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(A) ACOELOMATES: solid body, no coelom cavity; eg Planaria (flatworms)

(C) COELOMATES: true coelom cavity; eg Annelids (segmented earthworms)

These slender worms have a complete digestive tube, which extends from the mouth all the way to the anus Their muscles run lengthwise through their entire bodies, which are nonsegmented (not divided into small segments) But there is no true coelom (cavity with an actual lining of mesoderm) between the muscles and the digestive tube There is, instead, a pseudocoelom cavity that is lled with uid, but is not lined with mesoderm Finally, we turn to the coelomates The coelomates are commonly represented by the annelids (AN-eh-lids) the worms with many little rings of muscles encircling their bodies The most familiar annelid is the common earthworm (Figure 104, C) The earthworm is considered a segmented worm, due to the division of its body into numerous ring-like segments The uid- lled space surrounding its digestive tract is a true coelom, because

it is lined by cells from the mesoderm Earthworms are probably the most frequently studied of all coelomates in introductory biology classes But human beings, like earthworms, are also coelomates! In both annelids (such as earthworms) and humans, the uid within the coelom acts as a valuable shock absorber, cushioning the internal organs from blows hitting the outer body surface It also moistens and lubricates them, reducing their friction and rubbing during body movements

Closely related to the segmented worms is another group of coelomates, the mollusks (MAHL-usks) This phylum of mollusks consists of a huge number of more than 100,000 di erent species of invertebrates with soft bodies (mollusc) that are nonsegmented Mollusks include clams, snails, oysters, squids, and octopuses What could such more-or-less round-shaped organisms without segments possibly have in common with the segmented annelid worms you might well ask The answer is that both annelids and mollusks probably have a common ancestor in the Fossil Record, and they were the rst two groups of animals to develop a true uid- lled coelom cavity, lined with mesoderm cells

Because they have a soft body, many kinds of mollusks are protected by a rock-hard, calcium-rich shell Those early mollusks who developed this adaptation would obviously have enjoyed a much greater protection from hungry predators Squids and octopuses have either a much-reduced shell, which is mostly internal, or they have lost their shells entirely Despite their obvious di erences in degree of protection by shells, the mollusks all share a common basic body plan The basic plan is shown in Figure 105 The three main parts of any mollusk are the mantle, visceral (VIH-sir-al) mass, and foot The mantle is a little cloak or covering on the back of the mollusk The covering tissue within the mantle produces the hard shell in many species And since it is on the back or superior surface of the mollusk, the mantle functions in respiration, exchanging gases with the surrounding air or water In hundreds of mollusk species, there is a mantle cavity also present below the mantle itself The mantle cavity contains, in turn, a set of gills Since the mantle is hardened by an additional shell, it has a reduced

surface area available for di usion of oxygen (O2) into the blood, and di usion of carbon dioxide (CO2) out This reduced surface area available for respiration is compensated for by the gills, which assist with this critical job The visceral mass is the main soft body of the mollusk, which contains the major viscera (VIH-ser-ah) or guts (internal organs) The coelom cavity is also found within the visceral mass It holds the heart of the organism The foot of the mollusk is the inferior portion of the visceral mass, which, somewhat like a human foot, is large and eshy and at The foot pushes against the ground or sea bottom, propelling the mollusk forward In addition to their basic body plan, many species of mollusks are classi ed as bivalves (BUY-valves) This is because they have two (bi) shells hinged tightly together, like a valve This double-valve, when opened, gives the organisms bilateral symmetry Bivalves include such well-known invertebrates as the clams, oysters, mussels, and scallops

Echinoderms: Such A Prickly Skin May Make You A Star!

Bivalves have bilateral (mirror-image) symmetry, while adult star sh possess radial symmetry Various species of star sh, along with the sea urchins (URchins), belong to a phylum of invertebrates called the Echinoderms (ih-KY-