Coelomates: Animals with Internal Body Cavities

Coelomates: Animals with Internal Body Cavities

Coelomates are animals that have internal body cavities, or coeloms. Humans are coelomates, we have an abdomenal cavity (digestive organs, some of the excretory and reproductive organs) and a thoracic cavity (heart and lungs). Coelomates also form a variety of internal and external skeletons. External skeletons and coeloms appeared during the Cambrian-Ordovician time, These skeletons offered several advantages to their producers:

1. Secretion of a mineral shell that allowed the animal to use the shell as a mineral repository.
2. Protection from drying out in the intertidal zone during low tides.
3. Protection from predators.
4. Sites for anchoring muscle attachments, offering new patterns of locomotion and increased strength.

Phylum Mollusca: Clams, Scallops, and Squids

The phylum Mollusca contains over 100,000 species with a variety of body forms and lifestyles. In mollusks, the coelom is reduced and limited to the region around the heart. The Mollusk body first appeared during the Cambrian Period. All mollusks have:

1. a visceral mass containing internal organs, including the digestive tract, paired kidneys, and reproductive organs.
2. a mantle that surrounds but does not cover entirely the visceral mass and secretes a shell (if one is present). The mantle also contributes to formation of gills or lungs.
3. a head/foot region containing sensory organs and a muscular structure (foot) used for locomotion. The foot is a muscular structure used for locomotion, attachment to a substrate, food capture, or a combination of functions.
4. A radula is an organ that bears many rows of teeth and is used for grazing on food.
5. The nervous system consists of several ganglia connected by nerve cords.

Most mollusks have an open circulatory system: a heart that pumps hemolymph through vessels into a hemocoel. Blood diffuses back into the heart and is pumped out to the body again. Some mollusks are slow moving, and have with no head, while others are active predators that have a head and sense organs.
Classification of the Mollusca
The Class Polyplacophora

Chitons, shown in Figure 3, are in the taxonomic class Polyplacophora. They have a shell consisting of eight overlapping plates. A ventral muscular foot is used for creeping along the substrate, or for clinging to rocks. A chiton feeds by scraping algae and other plant food from rocks with its well-developed radula.
The Class Gastropoda

The class Gastropoda includes snails, terrestrial slugs, whelks, conchs, periwinkles, sea hares, and sea slugs. Most gastropods are marine, although some there are freshwater and terrestrial forms. Many gastropods are herbivores that use their radula to scrape food from surfaces. Carnivorous gastropods use their radula to bore through a surface, such as a bivalve shell, to obtain food. Most gastropods have a well developed head with eyes and tentacles projecting from a coiled shell that protects the visceral mass, as shown in Figure 4. The coiled shells of gastropods are often quite commonly found as fossils. One genus, Turetella, occurs in such quantities in a type of rock that the rock is known as "Turetella agate". However, not all gastropods have shells, the nudibranchs (sea slugs) and terrestrial slugs lack shells.

In aquatic gastropods, gills are found in the mantle cavity; in terrestrial gastropods, the mantle is richly supplied with blood vessels and functions as a lung when air is moved in and out through respiratory pores. Terrestrial gastropod embryonic development does not go through a swimming larval stage, as is the case in aquatic gastropods. For terrestrial snails, their shell not only offers protection but also prevents desiccation (drying out). The muscular foot contracts in peristaltic waves from anterior to posterior causing secretion of a lubricating mucus.

Terrestrial gastropods are hermaphroditic. In premating behavior, they meet and shoot calcareous darts into each other's body wall. Each inserts a penis into the vagina of the other, providing sperm for future fertilization of eggs. Eggs are deposited in the soil and development proceeds without formation of a larval stage, a common theme in some terrestrial invertebrates. Hermaphroditism assures that any two animals that meet can mate, which is especially useful in slow-moving animals.
The Class Bivalvia

The class Bivalvia consists of clams, oysters, mussels, and scallops. Members of this class have two-part shells that are hinged and closed by powerful muscles. The presence of shells in this group has yielded an impressive fossil record. The bivalves have no head, no radula, and little cephalization, as can be seen in Figure 5. Clams use their hatchet-shaped foot for burrowing; mussels use it to produce threads to attach to objects. Scallops can both burrow or swim. A rapid closing and opening of their two valves releases water in spurts.

The bivalve shell is secreted by the mantle. The shell is composed of protein and calcium carbonate with an inner layer of pearl. Pearls form as layers of shell-forming material deposited about a foreign particle lodged between the mantle and the shell. A compressed muscular foot projects down from shell. By expanding the tip, the foot pulls the body after it. Beating cilia of the gills cause water to enter the mantle cavity by way of the incurrent siphon and to exit by way of the excurrent siphon. While cilia of gills move water through the mantle cavity, gills also capture particles in water and move them toward the mouth. From the mouth food goes to the stomach, then to the intestine, which passes through the heart and ends at the anus.

Bivalves, like other mollusks, have an open circulatory system. Their nervous system consists of three pairs of ganglia. Two excretory kidneys below the heart remove ammonia waste from the pericardial cavity into the mantle cavity, from which it will leave the body.

Sexes in class Bivalvia are separate. The gonad is located around the coils of the intestine. Certain clams and annelids have the same type of larva, hinting at a possible evolutionary relationship between the two groups.
Fossil Record of Bivalves

Since they have hard shells, the fossil record of this class is remarkably good. Hard shells (or hard parts) are one of the features that make an organism a better candidate to become a fossil. Gastropods, another class of the phylum Mollusca, also become more prevalent in the Ordovician seas. Ordovician deposits yield snails, as well as large, sedentary gastropods such as Maclurites,

During the Mesozoic era, bivalves became more abundant and important parts of reefs. They would would remain important parts of the marine fauna throughout the Mesozoic. These bivalves, specifically the rudistids began to play a larger role in reef formation. Rudistid reefs are so named because the rudistid bivalves were the dominant reef-forming organisms. Biodiversity was reduced by mass extinctions at the end of the Triassic and Jurassic periods of the mesozoic era.

The bivalves recovered from the Jurassic extinctions and again became major reef-formers in the numerous shallow marginal seas that encroached onto the continents during the Cretaceous,
The Class Cephalopoda

The class Cephalopoda (literally "head-footed") includes squids, cuttlefish, octopuses, and nautiluses (and extinct relatives, the goniatites, ammonoids, and ammonites). The presence of a shell in many representatives of this class has yielded an impressive fossil record.

Squids and octopuses can squeeze water from their mantle cavity out through a funnel), thus propelling them with a form of jet propulsion. Surrounding their head are tentacles with suckers that can grasp prey and deliver it to a powerful beak/mouth. Cephalopods in general have well-developed sense organs, including focusing camera-type eyes. Most cephalopods, especially octopuses, have well-developed brains and show a capacity for learning. Nautiluses are enclosed in shells, squids have a shell that is reduced and internal, while octopuses lack a shell.

Squids and octopuses possess ink sacs from which they squirt a cloud of ink, as a means of escaping predators. Squids possess a vestigial skeleton under the mantle, called the pen, which surrounds the visceral mass. A squid has three hearts, one pumps blood to internal organs; two pump blood to the gills in the mantle cavity. Gonads make up a large portion of the visceral mass. Cephalopds have separate sexes. Spermatophores contain sperm, which the male passes to the female mantle cavity by way of one of his tentacles. After fertilization, eggs are attached to the substratum in strings containing up to 100 eggs.
Fossil Record of Cephalopods

During much of their evolutionary history cephalopods possessed a hard shell. Their abundance, the presence of a shell, and the environments they lived in led to an excellen fossil record for the group.

The Ordovician period saw the exolution and spread of coiled, swimming cephalopods. This group, the nautiloids (), resembles somewhat their living distant relatives the chambered nautilus and squids.

During the Devonian period Cephalopods ammonoid group known as the goniatites appeared. These coiled, chambered nautiloids, some of which are shown in Figure 11, left a great many fossils, some of which are quite aesthetically appealing.

The ammonoids underwent three separate diversifications from a nautiloid-like stock. In each case the fold pattern of sutures became more complex. These sutire patterns are fantastic characters for identifying species, making ammonoids excellent index fossils. The first of these occurrences was the goniatites, a group that ranged from the Devonian to the Permian. The ceratites are a Triassic group, while the last group, the ammonites ranged from the Triassic to the Cretaceous. Ammonoids finally went extinct in the great end-of-the-Cretaceous extinction. Nautiloids are represented today by the Nautilus. Differences between the groups The cephalopods recovered from the extinction of the goniatites at the close of the Paleozoic era and developed a remarkably similar group, the ceratites. These coiled, chambered animals, such as the one shown in Figure 12, had slightly more complex sutures than did the goniatites. However, the Ceratites also went extinct during the middle Mesozoic era.

During the Jurassic, the cephalopods once again produced a new coiled, chambered form, the ammonites. Suture patterns of these forms were even more elaborate than those found in the Triassic ceratites. The belemnites were straight-shelled cephalopods with elaborate suture patterns. Both ammonites and belemnites survived the Jurassic extinctions and flourished during the Cretaceous period.

Ammonites continued their dominance, as did their relatives the straight-shelled belemnites. Modern teleost fish appeared during the Cretaceous and may have competed for the same prey as the ammonites. The teleost fish were apparently stronger and swifter swimmers than the fish of the Jurassic. Some paleontologists speculate that the extinction of ichthyosaurs during the Cretaceous may have been hastened by the rise of these new faster fish that would have been difficult for the ichthyosaurs to catch and eat.

Baculites, a genus of straight-shelled cephalopods, was particularly abundant in the Cretaceous seas. Note the elaborate suture patterns in the fossil specimen below. Common fossils in the Cretaceous rocks, the cephalopods were major victims (along with the gastropod group the rudistids) of the terminal Cretaceous extinction event. Squid, octopus, and the chambered nautilus are the remnants of this once flourishing group of molluscs.
Phylum Annelida: Segmented Worms

The phylum Annelida contains segmented worms (The development of segmented bodies allowed the formation of specialized functions in different segments. Annelids have an enlarged coelom to accommodate more complex internal organs. The well-developed, fluid-filled coelom and the tough integument act as a hydrostatic skeleton. There are about 12,000 marine, freshwater, and terrestrial species usually divided into three taxonomic classes. Similarities of larval forms to Mollusks suggest annelids share an common ancestral group.

Annelids have a closed circulatory system with blood vessels running the length of the body and branching into every segment. Closed circulatory systems are more efficient than open ones for moving materials within a body. The annelid nervous system consists of a brain connected to a ventral solid nerve cord, with a ganglion in each segment. Annelids have a complete digestive system that include a pharynx, stomach, intestine, and accessory glands. Excretory nephridia in each segment collect waste material from coelom and excrete it through the body wall.
Classification of the Annelida
The Class Polychaeta

Most annelids belonging to the taxonomic class Polychaeta are marine and possess parapodia and setae. Parapodia are paddlelike appendages used in swimming that also serve as respiratory organs. Setae are bristles, attached to parapodia, that help anchor polychaetes to their substratum and also help them move. Clam worms, such as Nereis, are active predators. Many have well-developed cephalization, with a head having well-developed jaws, eyes, and other sense organs. Sedentary filter feeders possess tentacles with cilia to create water currents and to select food particles. Only during breeding do polychaetes have reproductive organs. Polychaet zygotes develop into a type of larva similar to that produced by marine clams.
The Class Oligochaeta

The class Oligochaeta includes earthworms, that tend to have their few setae protruding in clusters directly from their body. Earthworms have poorly developed heads or parapodia. Locomotion is by coordinated movement of the body muscles and assistance of their setae. When longitudinal muscles contract, segments bulge and setae protrude and anchor into the soil. Circular muscles contract, causing the worm to lengthen, setae are withdrawn and the segment moves forward.

Earthworms reside in moist soil where a moist body wall facilitates gas exchange. Earthworms are scavengers that extract organic remains from the soil they eat. A muscular pharynx draws food into the mouth. Ingested food is stored in a crop and ground up in a muscular gizzard. The dorsal surface of the intestine is expanded into a typhlosole that allows more surface area for digestion. External segments correspond to internal septa (walls) separating each body segment.

The earthworm excretory system has coiled nephridia tubules in each segment with two openings: one is a ciliated funnel that collects coelomic fluid, and the other is an exit in the body wall. Between the two openings, the coiled nephridia tubule allows removal of waste materials from blood vessels.

Red blood is moved anteriorly by a dorsal blood vessel and pumped by five pairs of hearts (sometimes referred to as aortic arches) to a ventral vessel. Earthworms are hermaphroditic, having both testes with seminal vesicles, and ovaries with seminal receptacles. Mating involves the worms lying parallel to each other facing opposite directions and exchanging sperm. Each worm possesses a clitellum that then secretes a mucus, protecting sperm and eggs from drying out. Embryonic development lacks a larval stage.
The Class Hirudinea

The class Hirudinea includes leeches. Most are freshwater, but a few are marine or terrestrial. Each body ring has several transverse grooves. Leeches possess a small anterior sucker around the mouth and a larger posterior sucker. Although some are free-living predators, most are fluid feeders. Bloodsuckers keep blood from coagulating by hirudin, an anticoagulant in their saliva. Leeches were commonly used in early medicine to "bleed" the patient.
Phylum Arthropoda: Segmented Bodies with Segmented Appendages

The phylum Arthropoda contains animals with segmented appendages on their body segments. Arthropods occupy every habitat, and are in many respects the most successful animal group on Earth. There are conservatively over 1 million species of living arthropods. Biologist E.O. Wilson estimates there are 10 million species, 9 million of which are arthropods. Certain groups of arthropds have extremely complete fossil records.

Arthropod features that have contributed to their success include:

1. A hard exoskeleton, a strong but flexible outer covering composed primarily of the carbohydrate chitin. This functions in protection, attachment for muscles, locomotion, and prevention of desiccation.
2. Presence of jointed appendages. Trilobites, which flourished during Cambrian Period and were important animals in marine ecosystems for the remainder of the Paleozoic Era, had a pair of appendages on each body segment. Modern arthropod appendages are specialized for walking, swimming, reproduction, etc. These modifications account for much of the diversity and success of arthropods.
3. A complex nervous system with a brain connected to a ventral solid nerve cord. The head bears various sensory organs. Compound eyes have many complete visual units, each of which collects light independently. The lens of each visual unit focuses the image on light sensitive membranes of a small number of photoreceptors within that unit. In simple eyes (like our own), a single lens brings the image to focus into many receptors, each of which receives only a portion of the image.
4. A unique respiratory system that employs a variety of respiratory organs. Marine arthropods utilize gills composed of a vascularized, thin-walled tissue specialized for gas exchange. Terrestrial forms have book lungs (e.g., spiders) or tracheae. (e.g., insects). Book lungs are invaginations to serve in gas exchange between air and blood. Tracheae are air tubes that serve as ways to deliver oxygen directly to cells.
5. A complex, yet adaptable, life cycle. Metamorphosis is a drastic change in form and physiology that occurs as an immature stage becomes an adult. Metamorphosis contributes to the success of arthropods because the larval stage eats food and lives in environments different from the adult; reducing competition between immature and adults of a species. Reduction in competition thus allows more members of the species to exist at one time.

The arthropod body consists of three major collections or zones of body segments:

1. head
2. thorax
3. abdomen

Classification of Arthropods

Due to their great diversity of appendages, lifestyles, and other features, arthropods are usually separated into several subphylums.
The Subphylum Chelicerata

The subphylum Chelicerata includes spiders, scorpions, ticks, mites, horseshoe crabs, etc. The first pair of appendages are chelicerae, second pair are pedipalps, and the next four pairs are walking legs. Chelicerae are appendages that function as feeding organs. Pedipalps are feeding or sensory in function; although in scorpions, they are large pincers. All appendages attach to a cephalothorax, a fusion of the head and thoracic regions. The head lacks antennae, mandibles, or maxillae appendages.
The Class Merostomata

The class Merostomata contains the extinct "sea scorpions" (or eurypterids) and the extant (living) horseshoe crabs. Eurypterids are extinct, but were important elements of faunas 200-500 million years ago during the Paleozoic Era. Some were huge, reaching a length of over 10 feet. Some eurypterids may have been amphibious, emerging onto land for at least part of their life. Horseshoe crabs are an ancient group consisting today of only 5 species. Members of this class have a large shield that covers the cephalothorax. The compound eyes are reduced. The second pair of appendages, the pedipalps, resemble walking legs. They have a long, spike-like appendage called a telson that projects from the rear of their bodies. Respiration is via book gills (precursors to book lungs?).

The horseshoe crab genus Limulus is a familiar sight along the east coast of North America. The anterior shield is a horseshoe-shaped carapace with two compound eyes. The long, unsegmented telson projects to the rear. They possess book gills that resemble the pages in a book. Limulus is considered a living fossil due to its great similarity to fossil forms from the Paleozoic Era.
The Class Arachnida

The class Arachnida includes over 60,000 described species (and most likely a very large number of as yet undescribed ones) of spiders (around 35,000 species), mites and ticks (25,000 species), scorpions (1200 species), and other forms. Nearly all arachnids are terrestrial.

Arachnids have a cephalothorax covered with a carapace-like shield. The abdomen may be segmented or unsegmented. Appendages on the abdomen are absent or modified, for example forming the spinnerets of spiders. Respiration is via tracheae or book lungs.

Scorpions are arachnids. They are the oldest terrestrial arthropods known from fossils. All scorpions are nocturnal and spend most of the day hidden under a log or rock. Their pedipalps are large pincerlike appendages, and their abdomen ends in a stinger containing venom.

Ticks, shown in Figure 17, are parasites that suck blood and sometimes transmit diseases. Chiggers are larvae of certain mites and feed on the skin of vertebrates.

Spiders, shown in Figure 18, have a narrow waist separating the cephalothorax from the abdomen. Spiders have numerous simple eyes rather than compound eyes. The chelicerae are modified as fangs with ducts from poison glands. The abdomen has silk glands used to spin a web to trap prey. Invaginations of the body wall form lamellae (pages) of the book lungs; air flows across the lamellae in the opposite direction from blood flow to exchange gases more efficiently.
The Subphylum Crustacea

The Subphylum Crustacea, , contains 30,000 mostly marine species. A few species live in freshwater. Lobsters, crabs, crayfish, shrimp, copepods, barnacles, and several other groups of organisms belong to this subphylum. All crustaceans possess two pairs of antennae, a pair of mandibles, a pair of compound eyes (usually on stalks), and two pair of maxillae on their heads, followed by a pair of appendages on each body segment. Crustacean bodies usually have a head, thorax, and abdomen. Crustaceans utilize gills for gas exchange.

Most crustaceans are free-living, but some are sessile and a few are even parasitic. Some crustaceans filter tiny plankton or bacteria from the water, while others are active predators. A few crustaceans scavenge nutrients from detritus.

Many species, including lobsters, crayfish, barnacles, and crabs are economically important (yum, yum). Krill, and a few other species, form the base of extremely important marine food chains. Still others are crucial in recycling nutrients trapped in the bodies of dead organisms.

The subphylum contains several taxonomic classes. We will focus on one, the class Malacostraca, which includes the shrimp, lobsters, etc.
The Class Malacostraca

The class Malacostraca is the largest taxonomic class of Crustaceans, having over 20,000 primarily marine species. Some malacostracans are freshwater, while others occupy diverse terrestrial habitats. Typical malacostracans include sowbugs, krill, and a very large order, the Decapoda, that contains many kinds of shrimp, crabs, and crayfish. Malacostracans typically possess a body with eight thoracic and six abdominal body segments, each bearing a pair of appendages. Class Malacostraca contains a number of economically significant species, such as edible lobsters, shrimp, crayfish and crabs. Many malacostracans contribute to plankton and as such are at the base of an immensely important marine food chain.
The Subphylum Uniramia

This subphylum contains arthropods that have unbranched appendages. The uniramian body has two or three tagmata, and an abdomen that has many segments. Appendages in the head region include paired antennae and mandibles, and also two pairs of maxillae. Gas exchange is by means of tracheae and spiracles. This subphylum include millipedes, centipedes, and insects.
The Class Chilopoda

This taxonomic class includes 20 families and more than 2500 species of centipedes, all terrestrial. Most centipedes are small, but a few can attain a length of up to 10 inches (25 cm). Centipedes have bodies are made up of a chain of many (up to 177) flattened segments. With the exception of the segment behind the head and the last body segment, each segment has a single pair of appendages (legs). The appendages of the first body segment have been modified to form large, poisonous fangs that are used to capture prey. The bite of a large centipede, however, can be painful to an adult and dangerous to a small child.
The Class Diplopoda

Millipedes, Figure 20, comprise this class containing some 8000 species. Bodies of members of this class are made up of numerous segments. Millipedes lack poisonous fangs and do not bite. Prerdators are discoraged by the millipede's rolling into a defensive ball. Production of poisonous or foul-smelling substances also serve to disuade any would be predators. Most millipedes are apt burrowing herbivores or scavengers.
The Class Insecta

Insects, Figure 21, are the largest group, with probably over one million identified and named species (and undoubtedly a greater number as yet unknown to us). Insects live in almost all terrestrial and freshwater habitats, with a few species living in the oceans.

Many insects have some thoracic appendages modified for flight, as shown in Figure 21, 22. Insects are important as pollinators for flowering plants, as well as for the damage they do annually to crops, and the diseases they transmit (malaria, some forms of encephalitis, Dengue Fever, the West Nile virus, etc.).

Insects display a wide huge variation in body styles, although there seems to be a size limit on the insect-style of body organization. Common features shared by most living insects include:

* body composed of three tagmata
* head
* thorax
* abodmen
* one pair of relatively large compound eyes
* usually three ocelli located on the head
* one pair of antennae on the head
* mouthparts consisting of a labrum, a pair of mandibles, a pair of maxillae, a labium, and a tonguelike hypopharynx
* two pairs of wings derived from outgrowths of the body wall
* three pairs of walking legs

Insects have a complete, complex digestive system. They exchange gases through a tracheal system, with external openings called spiracles dividing into finely branched tubules that carry gases directly to metabolizing tissues. Aquatic forms may exchange gases through the body wall or may have various kinds of gills. Excretion of nitrogenous waste takes place via Malpighian tubules. The nervous system of insects is complex, including a number of ganglia and a ventral, double nerve cord. Sense organs are complex and acute. In addition to ocelli and compound eyes, some insects are quite sensitive to sounds, and their chemoreceptive abilities are excellent.

Growth patterns are quite variable. Some insects hatch from eggs as miniature adults, which in turn shed their exoskeleton. Most insect species have newly hatched young that are completely different in appearance from adults. These larval forms usually live in different habitats, eat different foods, and look completely different from their adult stages. When larval growth is completed, the larva stops feeding and builds a case or cocoon around itself. In this nonfeeding condition (pupa or chrysalis) the larva undergoes a complete transformation or "metamorphosis" of its body form, eventually emerging as a fully-formed adult.

Insects are very valuable to us. While insects eat our food, feed on our blood and skin, contaminate our dwellings, and transmit diseases, we could not exist if thety were not here. Insects are a vital part of our ecosystem, functioning in:

* pollination of many flowering plants
* decomposition of organic materials
* recycling of carbon, nitrogen, and other essential nutrients
* control of populations of harmful invertebrate species (including other insects)
* direct production of certain foods like honey
* manufacture of useful products such as silk and shellac

So, have you hugged a bug today?
Deuterostomes and Protostomes

Protostomes (mollusks, annelids, and arthropods) develop so that the first opening in the embryo is the mouth (protostome = first mouth). Protostomes are bilaterally symmetrical, have three germ layers, the organ level of organization, the tube-within-a-tube body plan, and a true coelom. The coelom, a body cavity between the digestive tract and body wall completely lined by mesoderm allows the digestive system and body wall to move independently. Because of this, internal organs can be more complex. Coelomic fluid assists respiration and circulation by diffusing nutrients, and excretion by accumulating wastes. This fluid functions in place of several organ systems in higher animals such as mammals. The coelom may serve as a storage area for eggs and sperm, facilitating development of these gametes within the animal body. Coelomic fluid protects internal organs and also serves as a hydrostatic skeleton. Protostomes develop their embryo by spiral cleavage, as shown by Figure 23.

Deuterostomes (echinoderms and chordates) develop the anus first, then the mouth at the other end of the embryo. Deuterostomes are coelomate animals these embryological characteristics:

* Radial cleavage (Figure 22) in embryonic cell division: the daughter cells sit on top of previous cells.
* Fate of cells is indeterminate; if embryonic cells are separated, each one develops a complete organism.
* The blastopore is associated with the anus, and the second embryonic opening is associated with the mouth.

All text contents ©1995, 1999, 2000, 2001, 2005 by M.J. Farabee. Use for educational purposes is encouraged.

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