3o6 



PHYLUM ECHINODERMA 



dividual. In some forms of Cucumaria planci the body divides by 

 stricture, torsion, or stretching into two or three equivalent parts, each 

 of which may regenerate the whole. In this case the autotomy seems 

 to be reproductive. 



The worm-like body is often regular in form, with five 

 equidistant longitudinal bands, along which tube-feet 

 emerge. But three of these " ambulacral areas " may be 

 approximated on a flattened ventral sole, leaving two on 

 the convex dorsal surface, and there are other modifica- 



Fig. 165. — Spicules of Holothurians.— After Semon. 



The series 1-6 shows stages in the development of an anchor and a plate 

 in a Synapta. The series A-E shows stages in the development of 

 a wheel in Chiridoia, a Synaptid. Miscellaneous forms, ni-p. 



tions of form. In many cases the tube-feet are modified 

 into pointed papillae. 



The body wall is tough and muscular, consisting of 

 epidermis, dermis, and circular muscles, and there are 

 paired longitudinal muscles along each radius. A skeleton 

 is represented by scales, plates, wheels, and anchors of lime 

 scattered in the skin, and by plates around the gullet and 

 on a few other regions. 



The nervous system consists of a circumoral ring in 

 which the five radial nerves running in the ambulacral areas 

 unite, and from which nerves to the tentacles arise. The 

 ring and the radial nerves are sunk below the skin, 

 Coelomic nervous tissue is developed on the perihaemal 



