330 



ECIIINODERMA 



type. In other words, the echinoderms have descended from 



bilateral, possibly worm-like, ancestors. 



The structure of the integument gives these animals a charac- 

 teristic appearance. In the mesoderm under the 

 epithelium calcareous jjlates arise, forming a body 

 armor or test, and since these are usually pro- 

 duced into spines, they have given the name 

 Echinoderma, spine skin, to the grouj). This 

 mesodermal skeleton at times becomes degenerate, 

 as in the Holothurians (it rarely entirely disappears 

 as in PelagotJiuria), but even then shows itself 

 as spicules and 'wheels' of lime. The spha3ridia 

 and pedicellaria (fig. 303) — not always present — 

 are characteristic appendages of the integument. 

 The first are sense organs; the latter are usually 



stalked forcei^s-like grasping structures with calcareous skeleton. 



In life they are active and apparently either clean the skin or are 



defensive. 



Ftg. .303. — Pedicel- 

 laria, closed and 

 open 



Certain plates possess a morphologioal interest since they appear early 

 in many larvae, and in the jidults of different classes can be recognized in 

 similar positions. In the neighborhood of the arms are five basalia, inter- 

 radial in position, farther five radialia ('apical skeleton') and five inter- 

 radial ' oralia' around the mouth. 



Not less characteristic than the skeleton is the ambulacral (or 

 water-vascular) system (fig. 304). This begins usually externally 

 m and then ordinarily by a calcareous 



]>late, the madreporite, which is 

 perforated with fine pores and 

 serves for the entrance of sea water. 

 The water passes into a canal 

 which, on account of its calcified 

 walls in the starfish (fig. 305), is 

 called the stone canal and leads 



Fig. 304. 



Fig. 30."i. 



Fig. .301.— Water-vascular system of starfish (oris.). 



radial canal; m, madreporite; *», radial nerve, 



erverine; s, stone canal; (.racemose vesicle. 



beneath it the n__ 

 Fig. 3(1.') — Transverse 

 Ludwig.) 



ft, ampulla? ; (i'», ambulacra; 

 lu Polian vesicle ; /•, ring can; 



ng canal, 



section of stone canal of Aflrapcclfn aumntiacus. (After 



