330 



ECHINODERMA 



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 plates 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 group. This 

 mesodermal skeleton at times becomes degenerate, 

 as in the Holothurians (it rarely entirely disappears 

 as in Pelagothurid), but even then shows itself 

 as spicules and ' wheels ' of lime. The sphaeridia 

 and pedicellaria (fig. 303) not always present 



Fl ia'ria! 3 ciosed di and are characteristic appendages of the integument. 

 P en The first are sense organs; the latter are usually 



stalked forceps-like grasping structures with calcareous skeleton. 



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



defensive. 



Certain plates possess a morphological interest since they appear early 

 in many larvae, and in the adults 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 



and then ordinarily by a calcareous 

 plate, 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. 305 



FIG. 304. Water- vascular system of starfish (orig.). a, ampullae ; a/>, ambulacra ; c, 

 radial canal; rn, madreporite; n, radial nerve; p, Polian vesicle; r, ring canal, 

 beneath it the nerve ring; s, stone canal; f, racemose vesicle. 



FIG. 305 Transverse section of stone canal of Astropecten aurantiacus. (After 

 Ludwig.) 



