ORAL AND APICAL SYSTEMS OF THE ECHTNODERMS. 185 



These galleries enclosed between the super- and sub- 

 ambulacral plates lodged, I believe, not only the ambulacral 

 grooves but also the water vessels. In the recent Crinoids 

 and Starfishes these two are separated in each arm by a 

 membranous partition containing several structures, which 

 need not be considered here. Around the mouth the grooves 

 expand into a peristomial area, beneath which is the water- 

 vascular ring. The subtegminal galleries of Actinocrinus 

 open into what Wachsmuth^ calls an " annular' vessel," com- 

 posed of minute interlocking plates, and resting on the 

 convoluted digestive organ. This ring has five small open- 

 ings leading into the subtegminal galleries, and alternating 

 with these on the lower side of the ring there are five other 

 small openings, which Wachsmuth supposes to have been 

 " in connection with organs of the interradial system (com- 

 municating perhaps with a circulatory system)." 



This " annular vessel " represents, I believe, both the 

 peristomial area of our recent Crinoids and the subjacent 

 water-vascular ring, which is only separated from it by mem- 

 brane. It is easy to understand the nature of the inter- 

 radial openings on its floor. I imagine them to be the 

 openings of the small tubes which, in the recent Crinoids, 

 depend from the water-vascular ring into the body cavity. 

 According to Ludwig,- they open freely into it, and are to 

 be regarded as afferent vessels serving for the introduction 

 of water into the ambulacral system, and therefore as collec- 

 tively representing the sand canals of the other Echino- 

 derms. In the Ophiurids, however, there is not only a 

 sand canal extending from the madreporite to the water 

 vascular ring, but depending from this ring into the body 

 cavity there are a number of apparently csecal diverticula, 

 the vasa amhulacralia cavi of Simroth.^ The resemblance 

 between these and the so-called "^ Steincanale " of the Cri- 

 noids is very close, as pointed out by Huxley* and the two 

 sets of tubes appear to me to be truly homologous. Like 

 Greeff,^ I am not altogether satisfied {pace Ludwig) that 

 they actually open into the coelom in Comatula, while 

 Simroth believes them to be blind in Ophiactis. Hence I 

 cannot quite agree with Ludwig's view of their character. 



1 Loc. cit., p. 118. 



" " Crinoideen," loc. cit., p. 4S. 



' " Anatomie und Schizogonie der Ophiactis virens, Sars.," Zeitsch. f. 

 wiss. Zool.,' xxviii, p. 456. 



■* 'The Anatomy of Invertebrated Animals,' p. 586. 



* " Ueber den Bau der Crinoideen," ' Marburg Sitzungsberichte,' 

 No. 1, 1876, p. 22. 



