TYPE EOHINODERMA. 579 



the only one occurring with any marked degree of constancy 

 being the siphon (Fig. 265, si), a tube which arises from the 

 oesophagus and runs, closely applied to the intestine, to opea 

 again into it at the extremity of the oral coil. The function of 

 this structure appears to be respiratory, but it is to be noted 

 that it is wanting in all the members of one of the orders (the 

 Clypeastroidea) into which the group may be divided. 



The epithelial nervous system has the usual arrangement 

 consisting of a pericesophageal ring (Fig. 265, mr) and five 

 radial cords (rm). As in the Ophiuroidea, these latter struc- 

 tures have withdrawn themselves from the ectoderm and sunk 

 within the body-cavity, and accordingly there is to be found 

 an epineural sinus lying below the nerve-cords. Below the 

 nerve-ring, however, no sinus is to be found, and it seems 

 possible that it may have fused with the peripharyngeal space. 

 The extremity of each radial cord fuses with the ectoderm in 

 passing through the pore in the ocular plate, and is distrib- 

 uted to the walls of the terminal tentacle. A muscular 

 nervous system is present, consisting of five masses lying on 

 the aboral surface of the radial nerve-cords just where they 

 join the ring, and apparently having no direct connection 

 with each other ; they send fibres to the muscles of the mas- 

 ticatory apparatus, and are said to be wanting in those forms 

 which lack this organ. The visceral system consists of a 

 ring (an) lying near the margin of the. subperiproctal cavity 

 and imbedded in its wall. From the ring five branches arise 

 which pass to the walls of the ducts of the reproductive 

 organs. 



Sense-organs of various kinds have already been referred 

 to, such as the terminal tentacles of the hydrocoel canals, the 

 fascioles, and the sphaeridia. In addition to these, pigment- 

 spots occurring on the ocular plates have been regarded as 

 eyes, and somewhat complicated structures of a bright blue 

 color which occur abundantly over the surface of the test in 

 a species of Diadema have also been regarded as light-percip- 

 ient organs. 



As in other forms, the reproductive system consists of the 

 genital cords and the reproductive organs. The former have 

 their origin from a single cord, which is a hollow tube lined 



