OPHIUROIDEA. 201 



sinus (iPH) is thus cut off from the main portion. The brachial 

 portion is dilated segmentally over each pair of ambulacral 

 ossicles. The walls between these dilatations or chambers 

 are imperfect and are traversed by calcareous plates which 

 connect the ambulacral ossicles with the plates of the ambital 

 skeleton. There is a streak of epithelium carrying specially 

 strong cilia in the upper wall of the brachial continuation of 

 the peri visceral coelom (Fig. 140, 18). 



The water-vascular system is almost exactly as in Asteroids. 

 It consists of a circumoral vessel sending off a prolongation along 

 each arm. This gives off lateral branches to the tube-feet which 

 however are without ampullae and are purely sensory in function ; 

 it ends in the terminal tentacle. The first two pairs of tube- 

 feet are in relation with the mouth as oral tentacles (see p. 197), 

 and are supplied by canals which arise from the circumoral 

 vessel. There is a polian vesicle in each interradius except that 

 of the stone-canal. In Ophiactis virens, which has several stone- 

 canals, there are not only two or three polian vesicles in each 

 interradius, but also a number of tubular prolongations (canals of 

 Simroth) of the circumoral vessel which encircle the intestine 

 and penetrate between the generative organs. These tubes are 

 supposed to be respiratory in function, a view which is sug- 

 gested by the fact that the genital bursae are absent in this 

 species. 



The stone-canal (Fig. 145, 2), which however is without any 

 calcareous matter in its walls, passes ventralwards to open into 

 the ampulla (3), which corresponds to the whole of the axial 

 sinus of Asteroids and opens to the exterior by the pore-canal 

 (4) on the ventral surface of the disc, on one of the buccal shields 

 (oral plates). The opening of the water-pore is placed asymmet- 

 rically on the oral plate on an edge of it adjacent to a bursal 

 slit. As a rule there is only one water-pore, but in some species 

 of many genera of Ophiurae (Amphiura, Ophiolepis, Ophiopocus 

 Ophionereis, Ophiocnida), and in all Astrophytidae there are 

 several pores on the buccal shield concerned. In Trichaster 

 elegans, there are five stone-canals and five water-pores, 

 one in each interradius. In Ophiactis virens, which re- 

 produces itself by division, the stone-canals are repeated in 

 several interradii. In these cases of repetition of the stone- 

 canals, the young forms are said to have only one. 



