558 



INVERTEBRATE MORPHOLOGY. 



the lower layers of the ectoderm. Upon the aboral surface 

 of the oral ring aiid the radial nerves sections show distinct 

 bands of fibres separated from the ring and nerves by a 

 delicate layer of connective tissue ; these constitute the mus- 

 cular system of nerves (Fig. 254, mri), and their branches 

 appear to be supplied to the muscles of the body-wall and of 

 the ampullae and tube-feet. The aboral system is but feebly 

 developed when compared with that of the Criuoids. A trans- 

 verse section of an arm shows lying between the muscles of 

 the aboral surface and the peritoneal inesoderni a cord of 



FIG. 255. A STARFISH, Asteracantfiion, WITH THE INTEGUMENT OP THE DISK 



AND RAYS REMOVED TO SHOW THE INTERNAL STRUCTURE. 



a = anus. g reproductive organ. 



am = ampullae of tube-feet. I = liver cteca. 



ao ambulacral ossicles. M = inadrepori'te. 



c cardiac pouch of stomach. A-E = the five radii. 



nerve-fibres (Fig. 254, an), the five cords converging towards 

 the centre of the aboral surface of the body, the entire system 

 forming thus a five-rayed star. In position and general rela- 

 tions this system of nerve-cords is directly comparable to the 

 aboral system of the Crinoids, and may be regarded as 

 homologous with it. 



Special sense-organs are represented by the terminal ten- 

 tacles of the radial hydrocoel canals, which, as already stated, 

 retain a tentacle-like form and do not develop suckers at the 

 extremity. Their walls are richly supplied with nerves, and 



