272 



ECHIXODEEMATA. 



The former are moveably articulated to the knobbed tubercles on 

 the shell of the Sea-urchin, and are raised and moved laterally by 

 special muscles developed in a soft superficial dermal layer. The 

 pedicellarise (fig. 213) are stalked, prehensile appendages furnished 

 with two, three, or more rarely four jaws, which are continually 

 snapping together. They are especially collected around the mouth 

 of the Sea-urchin and on the dorsal surface of the Star-fish. 

 Small transparent bodies, splim-idia, are found in the living- 

 Sea-urchins, and probably have the value of sense organs. In the 

 Spatangidce, knobbed and ciliated bristles (clarula) are found upon 



the so-called 

 fascioles. 



St / The EcMno- 



dermata are 

 especially cha- 

 racterised by 

 the possession 

 of the peculiar 

 water - vascular 

 system and of 

 the distensible 

 ambulacra! feet 

 connected with 

 it (figs. 214, 

 215). This 

 ambulacral vas- 

 cular system 

 consists of a 

 circular vessel 

 su rroundin g 

 the oesophagus, 

 and of five 

 radial vessels 



projecting into the rays. These vessels have ciliated internal walls, 

 and contain a watery fluid. Very frequently a number of vesicles, 

 the Polian vesicles, are connected with the circular vessel, also a 

 number of racemose appendages, the significance of which is not 

 fully understood. In connection with the circular vessel there is 

 also a stone canal (in rare cases more than one are present), which 

 permits of communication between the sea water and the fluid 

 contents of the water vascular system. This canal, which is so 



Am 



FIG. 214. Diagram exhibiting the relations of the different systems 

 of organs in an Eckiims (after Huxley)- O, mouth; A, anus; Z, 

 teeth ; Z, lips ; Aur, auricula? of the shell ; re, retractor and pro- 

 tractor muscles of lantern ; Rg, circular ambulacral vessel ; P, 

 polian vesicle ; R', radial vessel of the same, with side branches 

 to the ambulacral feet (Am) ; Sc, stone canal; M, madreporic 

 plate; Sf, spine; P?, pedicellariif. 



