Invertebrates II

The Coelom and Coelomates

Evolutionary advantage of a coelom

  1. The advent of a coelom , the body cavity completely lined with a tough mesoderm gave those organisms that possessed it an evolutionary advantage.


  2. The coelom's lining is called the peritoneum. The mesentery, a folded membranous connection between body cavity and gut, originates from the peritoneum


  3. The mesentery holds body organs within the coelom in place but allows growth and movement.


  4. One basic advantage for those organisms with a coelom is greater control over locomotion. Soft-bodied animals use the coelomic fluids to from a hydrostatic skeleton as muscular pressure against the fluids gives the body a rigidity and provides a resistant base for the action used in burrowing.


  5. The coelomates include mollusks, annelids, arthropods &emdash; all protostomes; and echinoderms and chordates both deuterostomes.


  6. While both protostomes and deuterostomes form a coelom, the evidence from embryological development suggests convergent evolution, the coelom arising independently in these groups.

The Significance of the Protostome / Deuterostome Lines


  1. Protostomes exhibit (almost universally) great diversity, small bodies, extremely rapid reproductive cycles, and large population size; all characteristics of r-Selected species (pg 1207 Wallace). Learning is not a priority for survival.


  2. Although the early deuterostomes were not much different from their protostome cousins, they possessed great potential, which the brainy chordates amply exemplify.


  3. The first embryonic opening&emdash;the blastopore&emdash;forms the anus in deuterostome embryos and the mouth in protostomes. The two also differ in the origin of their skeletons. The exoskeleton of arthropods, shells of mollusks and bristles of annelids are nonliving, noncellular materials secreted by the ectoderm-derived epithelium. Most deuterostomes, however, fashion a living endoskeleton (often of hard calcium salts) from mesodermal cells.


  4. Both protostome and deuterostomes lines underwent major adaptive radiations during the Ediacara period of the late Precambrian.

The Protostome Line - Small Phyla

Lophophore Phyla - All are marine and many are extinct

Bryozoans (Ectoprocta) where once much more numerous than today. Bryozoans form branched encrustations along rocky shores. Their coelomate bodies possess a curved ridge called a lophophore covered by ciliated tentacles

Brachiopod shells resemble those of mollusks such as clams but their shells are oriented top to bottom rather than laterally (side to side) in relation to the body. The lophophore and tentacles are protected within the shell.


Phylum Molluska

Most are marine, some are fresh water species, and a few species are terrestrial (snails and slugs).


Mollusk means soft bodied. Mollusks are distinguished by a muscular foot (containing muscles and nervous units), which can be used for swimming, digging, holding on, or capturing prey.

The mantle, a fleshy organ is involved in respiration, either as a covering for the feathery gills of aquatic forms, or as a vascularized respiratory surface in terrestrial species. The mantle is also responsible for secreting the shell.

The circulatory system is open in most species (except the cephalopods). The veins do not return blood to the heart directly but end in spongy sinuses which surround it. The digestive system is typical of coelomates. The mouth often has a rasping tonguelike structure called a radula (Fig. 31.4) Osmoregulation is achieved by nephridia. The principle Classes of mollusks are the chitons, gastropods (snails), bivalves, and cephalopods (squid and octopus)

Phylum Annelida

Segmented worms are found in all environments.

The extreme segmentation found in the annelids is possibly an adaptation to locomotion necessary for burrowing. Their progressive, wavelike locomotion results from alternating contractions of circular and longitudinal muscles against a fluid filled, compartmentalized coelom. Segmentation is essential for hydrostatic skeletons.

Their circulatory system is closed within a complete set of arteries, veins, hearts (five sets of aortic arches) and capillaries. Hemoglobin carries oxygen but is free (not in membrane bound cells).

Osmoregulation is carried out by a pair of nephridia in each segment. The nephridia are a major advance over protonephridia found in the more primitive invertebrates because they not only remove nitrogen waste but, in terrestrial forms at least, they return water and important ions to the blood.

Earthworms are functionally hermaphroditic and can cross-fertilize.

The digestive system is typical of the tube-within-a-tube plan of coelomates. It consists of a muscular pharynx, a slender esophagus, a thin-walled crop or food receptacle, a muscular gizzard used for grinding ingested earth, and a long, straight intestine.

Annelids have well-developed regulatory and nervous systems.


Earthworms play an important role in soil ecology. By being continually loosened, stirred up, and aerated by the action of earthworms, soil is made more fertile. Earthworms also form a source of food for many animals, constituting the principal food of moles and shrews.


Earthworms are capable of burrowing with considerable speed, especially in loose soil; the bristles (setae) along the sides of the body are of great assistance in their movements. They provide anchorage to hold some segments fast while other parts of the body are extended or contracted. In burrowing, earthworms swallow large quantities of earth that often contain considerable amounts of vegetable remains. They are able to digest the nutritive matter of the soil, depositing or casting out the remains on the surface of the earth or in their burrows.

Phylum Arthropoda

This is the largest, most numerous, and most diverse group of animals on earth by far. There exists today anywhere from 1 to 10 million living species of Arthropods most of them insects.


Arthropod means jointed foot. This condition is the result of a tough exoskeleton make of chitin, and often hardened with calcium salts. It serves as a protective covering, and an attachment site for muscles and locomotion.

As they grow, arthropods must molt or shed their outer covering.

No arthropod is able to become very large, exoskeletons cannot be scaled up like an endoskeleton can, and their respiratory and circulatory systems are too inefficient to accommodate larger body masses. Nevertheless they have adapted to nearly every ecological niche possible.

Arthropods are segmented, but unlike the repeating units in annelid worms the arthropod segments are highly specialized, assigned specific tasks.

Arthropods have an enormous variety of diets living as: omnivores, herbivores, carnivores, scavengers, ectoparasites, and endoparasites. There success depends in great measure on their reproductive strategy which is in large part r-Selected.


Arthropods are classified into the following groups:

  • Trilobita (which includes the extensive fossil fauna of trilobites),
  • Arachnids or chelicerae (which includes the spiders, ticks, mites, and scorpions),
  • Crustaceans, Centipedes, Millipedes, and Insects.

Be familiar with the respiratory system terms (tracheae and spiracles for insects and book lungs for spiders.)


Phylum Onychophora

This is a small group which would normally be omitted but for the fact that it is considered to be an evolutionary link between the segmented worms and the arthropods.


The Deuterostome Line


The deuterostomes are mostly echinoderms and chordates. Chordates are covered separately.


Deuterostomes differ from the protostomes in a variety of ways, including

  • spiral cleavage,
  • secondary mouth development, and
  • enterocoelus development of the coelom (see pg 693 Wallace).

Many are K-selected species, which tend to be large animals with long life spans, continuous reproduction, and lengthy growth periods.


Phylum Echinodermata

The term echinodermata means "spiny skinned" referring to the bony ossicles found under their epidermis. Many are pentaradial, having 5 point symmetry.


Echinoderms are unusual in many ways.

Their spiny exterior is actually an endoskeleton covered by epidermis. It is made of calcareous plates (ossicles) that are secreted by the dermis which is mesodermal in origin.

The body of adult echinoderms is essentially a pentaradial configuration--a scheme found in no other phylum.

The larvae, however, are bilateral and develop by spiral cleavage--an important clue to their phylogeny.

Echinoderms show no obvious segmentation.

Another unusual feature is their water vascular system--a series of water-filled canals, which by hydraulic pressure help extend their numerous muscular tube feet. Above each tubular foot is a rounded sac, the ampulla, which acts like a squeeze bulb.

They lack a centralized nervous system. The starfish are famous for regenerating arms and other body parts after being cut up or attacked.