GENERAL ZOOLOGY 



disk and continuing along the oral surfaces of the arms and their branches 

 (Fig. 16.15). Currents maintained in these grooves converge upon the mouth 

 and carry into it the microscopic particulate' matter which constitutes the 

 food of the crinoids. Tube feet are present, but they lack ampullae and 

 terminal suckers; they have tactile functions and serve also in respiratory 

 exchange. As in holothurians, the water-vascular system of the adult crinoid 

 has no external madreporite; there are multiple "water tubes" or stone canals 

 which furnish communications between the ambulacral ring canal and the 

 coelomic cavity. 



The life cycle of the crinoids is best known for the feather star (Fig. 16.16). 

 It includes a barrel-shaped, ciliated larva which, though simplified, is com- 

 parable with the larvae of other echinoderms. After a free-swimming period, 

 this larva becomes attached and undergoes metamorphosis into a juvenile 

 form with a jointed stalk like that of the sea lilies. Eventually, the connection 

 between the stalk and the disk is broken, and the animal becomes free-moving. 

 This stalked phase in the life cycle of Antedon indicates that feather stars have 

 evolved from ancestors attached throughout their adult life. The fossil record 

 shows that stalked crinoids have had an extremely long evolutionary history, 

 dating from the end of the Cambrian period. Different crinoid groups have 

 flourished during successive geological eras; most of them were stalked forms, 

 and by far the majority have become extinct. The modern crinoid fauna is 

 dominated by the free-moving feather star type. 



The Echinoderm Body Plan 



When compared with the organization of other eucoelomate metazoans, the 

 body plan of a typical echinoderm appears in some respects highly aberrant. 

 Examination of details reveals, however, that there are more similarities than 

 differences between echinoderms and other eucoelomate animals. As is 

 typical of all, echinoderms are triploblastic and have a well-developed coelom. 

 Moreover, the arrangement of tissues in the body wall and in the wall of the 

 gut in an echinoderm is fundamentally like the arrangement in such other 

 metazoans as annelids and vertebrates. In each of these forms the body wall 

 is covered externally by an epidermis and internally by a peritoneum, and 

 between these two layers are muscular and connective tissues. The wall of 

 the digestive tract in each type consists of a covering peritoneum, a region 

 of muscular and connective tissues, and a lining of columnar epithelium. 

 Furthermore, these tissues arise from similar germ layers in the embryonic 

 development of these different forms, in ways which differ only slightly among 

 the three. Considering these and other general correspondences, it is clear 

 that the major dissimilarities stem from the fact that, unlike all other eucoe- 

 lomates, echinoderms are organized on a radially symmetrical plan. 



If, in addition, the bilateral larva and its probable evolutionary sig- 

 nificance are considered, the body of an adult echinoderm may be interpreted 



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