Coelomates are animals that have internal body cavities, or coeloms. Humans are coelomates, since we have an abdominal cavity containing digestive organs, some of the excretory and reproductive organs, and a thoracic cavity that contains the 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 offer 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.
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3. Protection from predators.
4. Sites for anchoring muscle attachments, offering new patterns of locomotion and increased strength.
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 organisation, 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.
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This fluid functions in place of several organ systems in the higher animals. The coelom may serve as a storage area for eggs and sperms, 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.
Deuterostomes (as exemplified by the echinoderms and chordates) develop the anus first, then the mouth at the other end of the embryo.
Deuterostomes are coelomate animals having these embryological characteristics:
i. Radial cleavage in embryonic cell division- the daughter cells sit on top of previous cells.
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ii. Fate of cells is indeterminate; if embryonic cells are separated, each one develops into a complete organism.
iii. The blastopore is associated with the anus, and the second embryonic opening is associated with the mouth.
Phylum Echinodermata:
There are 6000 species of echinoderms; all extant species are marine. The phylum Echinodermata includes the sea urchins, sea stars, sea cucumbers, and starfish. Most adults have radial symmetry, while their larvae are bilaterally symmetrical.
Echinoderms have an endoskeleton consisting of calcareous plates bearing spines. Radial symmetry appears to be an advantage to the mostly bottom-dwelling echinoderms, who can thus feed in every direction. Adult echinoderms have no brain. Members of the phylum have a water vascular system that powers their multitude of tube feet.
There are several taxonomic classes of echinoderms of varying familiarity to the general public.
They are:
i. Class Crinoidea:
The class Crinoidea includes about 600 species of crinoids, the stalked sea “lilies” and the motile feather stars. Their branched arms are used for filter-feeding and give the animals a flowerlike or plantlike appearance (hence the term sea lily). Crinoid stalks and blastoid heads are common fossils in certain parts of North America.
ii. Class Holothuroidea:
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The class Holothuroidea has 1,500 species of sea cucumbers. Holothuroideans have a long leathery body, and feed by tentacles located around their mouth.
iii. Class Echinoidea:
Scientists generally place about 950 species of sea urchins and sand dollars are in the class Echinoidea. Both sea urchins and sand dollars have spines that they use for locomotion, defense, and burrowing. Sea urchins have long, blunt spines. Sand dollars are flattened with a five- part flowerlike pattern of pores for skin projections.
iv. Class Ophiuroidea:
The class Ophiuroidea consists of approximately 2,000 species of brittle stars. Brittle stars have a central disk from which long, flexible arms radiate. These long arms allow them to move rapidly.
v. Class Asteroidea:
The class Asteroidea contains about 1,500 species of sea stars (commonly known as starfishes). Most starfish have a dorsoventrally flattened body. Starfish have a central disk to which five, or a multiple of five, sturdy arms are attached. Sea stars are common along rocky coasts where they eat clams, oysters, and other bivalves.
The five-rayed body has an oral (mouth) and aboral (upper) side. Spines project from the endoskeletal plate through the thin dermis. Pincer like pedicellarie keep the surface free from particles. Gas exchange is conducted by skin gills. On the oral surface, each arm has a groove lined with tube feet.
A sea star feeds by everting its stomach. It positions itself over a bivalve and attaches tube feet to each side of the shell. By working tube feet in an alternating fashion, it opens the shell open. Only a small crack is needed to insert its cardiac stomach into the prey. Stomach enzymes begin digesting the bivalve as it is trying to close its shell.
Partially digested food is then taken into the pyloric stomach for complete digestion. A short intestine opens at the anus on the aboral side. Each arm has a well-developed coelom containing a pair of digestive glands and male or female gonads. The nervous system is a central ring with radial nerves in each arm. A light-sensitive eyespot is at the end of each arm, facilitating coordinated but slow responses.
Locomotion depends upon the water vascular system. Water enters on the aboral side through the sieve plate, which is also known as the madreporite. Water passes through a stone canal to a ring canal and into the radial canals in each arm.
The radial canals feed into lateral canals extending into tube feet, each of which has an ampulla. Contraction of an ampulla forces water into the tube foot, expanding it; when the foot touches a surface, the center withdraws forming a suction and adhering to surfaces.
Echinoderms lack complex respiratory, excretory, and circulatory systems. Fluids within the coelomic cavity carry out the function of diffusing substances and gases. Gas exchange occurs across the skin gills and tube feet. Nitrogenous wastes diffuse through coelomic fluid and across the body wall. Cilia on the peritoneum lining the coelom keep the coelomic fluid moving.
Sea stars reproduce both sexually and asexually. If the body is fragmented, each fragment can regenerate a whole animal. Sea stars spawn and release either eggs or sperm at the same time. The bilateral larvae undergo a metamorphosis to become a radially symmetrical adult.
Phylum Hemichordata:
The Phylum Hemichordata includes about 90 species of acorn worms. Some acorn worms living on tidal mud flats have a proboscis, a collar, and a trunk. The dorsal nerve cord in the collar and trunk resemble the nerve cord of chordates. The pharynx below the collar has gill slits. Larva of hemichordates resembles the larva of echinoderms.
These facts cause some scientists to believe echinoderms and hemichordates share a common ancestor and that hemichordates and chordates are related by a common ancestor.
Hemichordates are classified into three classes, two with living members, one consisting of only fossil forms. The Enteropneusta includes the acorn worms; the Pterobranchia includes 20 species of colonial bottom dwelling worms. The fossil (extinct) group, the Graptolithina (graptolites), are common fossils in the Ordovician and Silurian. They have recently been placed in this phylum.
Phylum Chordata:
The Phylum Chordata includes about 45,000 species that occupy nearly all environments.
All chordates at some time during their life history have:
I. A notochord- a dorsal supporting rod located dorsally just below the nerve cord; it provides support and is replaced by the vertebral column in vertebrates.
II. A dorsal hollow nerve cord- a fluid-filled canal; spinal cord is protected by vertebrae.
III. Pharyngeal gill pouches- openings that function in feeding, gas exchange, or both.
These features are seen only during embryonic development in most vertebrates. Not all chordates are vertebrates. In the invertebrate chordates, fish, and amphibian larvae, pharyngeal gill pouches become functioning gills.
Terrestrial vertebrates have their pouches modified for various purposes; in humans, the first pouches become the auditory tubes, the second become tonsils, and the third and fourth pairs become the thymus and parathyroid glands.
Most chordates have an internal skeleton against which muscles work. Most have a post anal tail that extends beyond the anus; in some (like humans), this may only appear in embryos.
The evolutionary origin of chordates remains a mystery, although biochemistry and comparative embryology indicates echinoderms and chordates share a common ancestry. Although scanty, fossil finds from the Cambrian, suggests that chordates were present in the Burgess Shale deposits.
Invertebrate Chordates:
Not all chordates are vertebrates. Some chordates are invertebrates, lacking a vertebral column. In these invertebrate chordates, the notochord persists and is never replaced by the vertebral column.
i. Subphylum Urochordata:
The subphylum Urochordata contains 1,250 species of tunicates that have gill slits. Adults have a body composed of an outer tunic with an incurrent and excurrent siphon. When they are disturbed, tunicates tend to squirt water out. Water passes into a pharynx and out through numerous gill slits, the only chordate characteristic that remains in adults.
Microscopic particles adhere to a mucous secretion in the pharynx and are eaten. The larvae are bilaterally symmetrical and have the three chordate characteristics. Tunicate larva metamorphoses into the sessile adult. Beating of numerous cilia lining the inside of the pharynx creates a current to move water through a tunicate.
Some suggest larvae became sexually mature without developing tunicate characteristics; thus, a urochordate larva was ancestral to vertebrates; or a cephalochordate larva may have been ancestral to vertebrates.
ii. Subphylum Cephalochordata:
The lancelets have three chordate characteristics. The 23 species of lancelets are in the genus Branchiostoma in the subphylum Cephalochordata. Their elongated, lance-shaped body resembles the lancelet, a two-edged surgical knife. They inhabit shallow coastal waters; they lie partly buried in sandy substrates and filter feed.
Lancelets feed on microscopic particles filtered from the constant stream of water that enters the mouth and exits through the gill slits into an atrium that opens at the atriopore. Lancelets retain the three chordate characteristics as an adult. The notochord extends from head to tail, accounting for the name “Cephalochordata.” Lancelets have segmented muscles and their dorsal hollow nerve cord has periodic branches.
Vertebrate Chordates:
The vertebrates comprise a large group of chordates, and are subdivided into seven classes (3 classes of fish, amphibians, reptiles, birds, and mammals). Vertebrates have an internal skeleton of cartilage or bone, with vertebrae surrounding the dorsal nerve cord.
Subphylum Vertebrata:
The subphylum Vertebrata consists of about 43,700 species of animals with backbones. Vertebrates exhibit all three of the chordate characteristics at some point during their lives. The embryonic notochord is replaced by a vertebral column in the adult. The vertebral column is made of individual hard segments (vertebrae) surrounding the dorsal hollow nerve cord.
The nerve cord is the one chordate feature present in the adult phase of all vertebrates. The vertebral column, part of a flexible but strong endoskeleton, is evidence that vertebrates are segmented. The vertebrate skeleton is living tissue (either cartilage or bone) that grows as the animal grows.
The endoskeleton and muscles form an organ system (the musculoskeletal system) that permits rapid and efficient movement. The pectoral and pelvic fins of fishes evolved into jointed appendages that allowed vertebrates to move onto land. The skull, the most anterior component of the main axis of the vertebrate endoskeleton, encases the brain.
The high degree of cephalisation in vertebrates is accompanied by complex sense organs concentrated in the head region. Eyes developed as outgrowths of the brain. Ears were equilibrium devices in aquatic vertebrates that function as sound-wave receivers in land vertebrates.
Vertebrates have a complete digestive system and a large coelom. Their circulatory system is closed, with respiratory pigments contained within blood vessels. Gas exchange is efficiently accomplished by gills, lungs, and in a few cases, moist skin. Kidneys are efficient in excretion of nitrogenous waste and regulation of water. Reproduction is usually sexual with separate sexes.