286 ZOOLOGY FOR MEDICAL STUDENTS chap. 



A characteristic feature of the Echinoderms is the extremely compli- 

 cated subdivision of the coelomic cavity. In the primitive condition 

 the arrangement is simple, three pairs of coelomic sacs being budded off 

 the endoderm, but in the adult these have undergone curious lop-sided 

 developments in adaptation to the radial symmetry of the body so 

 that the original simple paired arrangement has become completely 

 unrecognizable. 



The formation of gonad is restricted to a single one of the coelomic 

 cavities, which takes on the form of a ring round the anus and sprouts out 

 into ten radiating tubes towards the bases of the arms. The gonad is in 

 the form of a thickening of the epithelium which extends along the various 

 portions of the genital coelome. The gonad never becomes functional 

 except at the tips of the ten radiating canals already mentioned, at each 

 of which it swells up to form a much lobed testis in the male or ovary 

 in the female. As it matures the ovary or testis sprouts out towards 

 the surface and develops an opening to the outside at the base of the 

 arm through which the eggs or sperms pass out. In an ordinary dis- 

 section the functional ovaries or testes are conspicuous, but all the rest 

 of the gonad and the cavities associated with it can only be worked out 

 by the examination of microscopic sections. 



The most remarkable development of the coelome is what is called 

 the hydrocoele — sometimes still called by the old name " water- vascular 

 system" — which has to do functionally with the movement of the 

 tube-feet. The hydrocoele of the fully developed starfish consists of 

 the following parts. Round the mouth opening there is a somewhat 

 pentagonal circum-oral ring prolonged along each arm as a radial canal 

 (Figs. 117 and 118, r.c). From the radial canal pass off numerous side 

 branches each of which communicates with a tube-foot and where it 

 does so is guarded by a valve that allows fluid to pass towards the 

 tube-foot but not in the opposite direction. The tube-foot (Figs. 117 

 and 118, t.f) is almost cylindrical in form and terminates in a sucking 

 disc : its walls are corjiparatively thin and contain muscle fibres which 

 run in a longitudinal direction. At its inner end the tube-foot is con- 

 tinued into a round bulb — the ampulla (Fig. 118, a) — which projects into 

 the main coelomic cavity of the arm. The ampulla possesses in its wall 

 circular muscle fibres by which it can be compressed. 



The functioning of the tube-foot takes place as follows. The muscles 

 of the ampulla contract and the coelomic fluid contained in it is forced 

 into the tube-foot which is consequently pushed out. On its tip coming 

 into contact with a solid object special muscle fibres in the terminal disc 

 contract and cause it to take a cup shape so that it adheres to the solid 



