viii ECHINODERMATATHE PSEUDOHSEMAL SYHTEM 447 



In the Comatulidce, in consequence of the absence of a stalk, the 

 arrangement is somewhat modified. We must imagine, however, that 

 only the internodes of the stalk are wanting, just as many whorl 

 joints being fused with one another and with the centrodorsal as there 

 are whorls of cirri. In this way the chambered sinus is enlarged by 

 the addition of the widened portions of the canal which were origin- 

 ally present in the whorl joints. The cirrus canals now rise direct 

 from the chambered sinus. 



As in the cirri of the stalked Crinoids, so also in those of the 

 Comatulidce, the canal is divided by a horizontal partition, and in these 

 latter animals also the partition of each cirrus canal is continued as far 

 as to the axis of the chambered sinus formed by the axial organ. The 

 chambered sinus of the Comatulidce, in sections taken in the direction 

 of the principal axis, appears therefore to be divided by these parti- 

 tions into as many spaces, consecutively superimposed, as there are 

 consecutive whorls of cirri. 



The five chambers of the chambered sinus, in close contact with 

 the axial organ, are produced orally for a short distance as ever- 

 narrowing canals accompanying the axial organ, and then end blindly. 



The system of the chambered sinus is, therefore, in adult Crinoids, 

 completely cut off from the rest of the ccelom. 



For the relations of this sinus to the apical nervous system, see the section deal- 

 ing with this latter, p. 460. 



VIII. The Pseudohsemal System. 

 (Radial Sinuses and Circular Sinus of the Schizoccel, Subneural Canals. 



The pseudohsemal system consists of canals which are closely 

 connected in the same manner in all Echinoderms with the oral 

 nervous system. As radial pseudohsemal canals, they accompany 

 the radial nerve trunks as far as to the ends of the radii ; and as 

 a pseudohsemal ring they accompany the nerve ring in its course 

 round the cesophagus. They always lie on the inner side of the nerve 

 trunks (that turned to the body cavity), between these trunks and the 

 water vascular trunks. The radial pseudohsemal canals give oft" side 

 branches which accompany the nerves of the tube-feet to their bases. 



The pseudohsemal canals are filled with a fluid which resembles 

 the ccelomic fluid. The intimate relation subsisting between these 

 canals and the oral nerve ring and the radial nerve trunks makes it 



o 



probable that they are specialised for the nourishment of these nerves. 

 It has also been conjectured that they, together with the epineural 

 canals, which we shall describe later, are of essential service in pro- 

 tecting the nerve trunks from being pressed or torn. 



In the Holothurioidea and Echinoidea the pseudohsemal system is 

 closed on all sides ; in the Asteroidea and Ophiuroidea, on the contrary. 



