662 - Heredity and Evolution 



Fig. 32-34. A common starfish (As- 

 terias forbesi), lying upside down. 

 Note the many tube feet, each tipped 

 with a suction disc. Four rows of these 

 feet protrude from the groove (am- 

 bulacral groove) that extends out into 

 each arm, on the lower surface. Also 

 note the many blunt spines that stud 

 the body wall. (By permission from 

 Integrated Principles of Zoo/ogy, by 

 C. P. Hickman. C. V. Mosby Co.) 



Perhaps more fundamentally, on the other 

 hand, the echinoderms may be regarded as 

 bilaterally symmetrical animals. All have a 

 bilaterally symmetrical larval stage, and even 

 in the adult there is a remnant of such sym- 

 metry in the placement of the madreporite 

 (Fig. 32-35). 



The water vascular system (Fig. 32-35) con- 

 stitutes another unique feature of Echino- 

 dermata. This water-filled system of tubular 

 channels, with muscular walls, connects with 

 a sievelike structure, the madreporite, on 

 the one hand, and with a large number of 

 tube feet on the other (Fig. 32-35). The 

 madreporite apparently serves to admit new 

 water from the surrounding sea, if the sys- 

 tem needs filling. The many tube feet co- 

 operate in achieving a slow and clumsy sort 

 of locomotion and in grasping food material. 

 The tube feet extend outward through the 

 body wall, entirely from the lower surface 

 in the case of the starfish (Fig. 32-3-1) and 

 related forms, but from other parts of the 

 body, in other echinoderms. 



The tube feet are highly extensible and 

 retractible and each may be equipped at its 

 end with a suction pad. When a foot con- 

 tracts, much of its content of water is forced 

 to flow back into other parts of the water 

 vascular system but especially into the bulb, 

 or ampulla, which lies directly above (Fig. 

 32-35). And conversely, when a tube foot is 

 being extended, the ampulla and neighbor- 

 ing parts contract, forcing the water to flow 



back into the lumen of the extending foot. 



No structure remotely comparable to the 

 echinoderm water vascular system is found 

 anywhere else in nature. Apparently the sys- 

 tem serves mainly for locomotion, but per- 

 haps it also helps to carry respiratory gases 

 and other substances. Obviously such a com- 

 plex and highly specialized system is not 

 well suited to the requirements of a land 

 animal — which probably accounts for the 

 fact that terrestrial Echinodermata have 

 never been evolved. 



The respiratory system among echinoderms 

 does not reach a very high state of develop- 

 ment. Many possess dermal branchiae, which 

 are small hollow blisters, protruding slightly 

 from the body wall near the bases of the 

 spines. Each is filled with coelomic fluid that 

 enters the vesicle through a narrow connec- 

 tion with the coelomic cavity. Thus oxvgen 

 can enter and carbon dioxide can leave the 

 body fluid through the delicate walls of the 

 branchiae. Other echinoderms display "respi- 

 ratory trees," which protrude from the pos- 

 terior end, and a few respire mainly through 

 the walls of the tube feet. 



Typically the central nervous system tends 

 to follow the pattern of the water vascular 

 system. A main nerve ring encircles the 

 mouth and a radial nerve extends out from 

 this ring into each of the body sections. More- 

 over, there is a system of fluid-filled tubes that 

 lies in close contact with the various parts 

 of the nervous system, but the functional im- 



