119 



propagate its species, none of its other parts making any farther advance. 

 Such an animal^ -svith some slight modifications, would not be very widely 

 different from a palaeozoic Crinoid. If the sarcodic body wall were to be 

 consolidated into a thin calcareous integument, with the mouth even with 

 the surface, the swimming appendages aborted, and the vent closed up, it 

 would resemble the cup of an Actinocrinus^ fig. T9 a. The lateral orifice 

 would then be both mouth and vent, as it is, at first (according to Prof. A. 

 Agassiz Seaside Studies, p.l*2o), in the embryo Asteracanthion BryUnus. 

 ' The ambulacral canals of Bijnnnaria are the homologues, in a general way, 

 of those which are found beneath the vault of Actlnocrinus, and extend 

 out into the grooves of the arms. If the ventral perisome of the Crinoid 

 were to be removed (the internal organs remaining undisturbed) the arrange- 

 ment disclosed would be that represented in fig. 79 a convoluted plate in the 

 centre with the canals radiating from it. The most striking difference is 

 the absence of the oesophageal ring. According to the organization of 

 Actinocrimis there could be no oesophagus at that point, and consequently 

 there is no ring. The convoluted plate represents the madreporic appara- 

 tus. The sucking feet of the Star-fish, most probably, represent the re- 

 spiratory tentacles that border the grooves of the Crinoids, but modified 

 into prehensile and locomotive organs. Blpinnaria and Actinocrtnus agree 

 in having the mouth in one of the interradial areas, and in the absence of 

 an orifice through the perisome at the ambulacral centre. These two 

 characters are embryonic and transitory in the Star-fish, but they were 

 permanent in most palagozoic Crinoids. 



In Codonites stdliformis (Pentremites stelUformis Owen and Shumard), 

 figs. 80, 81 the ambulacral centre c, is completely closed. Five minute 

 grooves radiate out to the extremities of the five angles of the disc. These 

 grooves are identical with those of Pentremites and Niicleocrinus and 

 were occupied by the ovarian tubes. The ambulacral canals of the true 

 Crinoids and of the Star-fishes are represented in a rudimentary condition, 

 in the species, by the hydrospires which open out to the surface through 

 the ten fissure- like spiracles, s. The oro-anal orifice is interradial. 0. 

 stelUformis in external form, the interradial position of the mouth, and the 

 closed ambulacral centre, resembles Blpinnaria and Actinocrinns, but 

 differs importantly in having its respiratory organs arranged in ten 

 separate tracts, all totally disconnected from each other. It is a lower form 

 than Actinocrinus, which in its turn is lower than Bipinnaria, and yet all 

 three are constructed on the same general plan. 



C. stelUformis, although much resembling a Pentrcmite, is a true 

 Cystidean. Its affinity to CjcJastcr vf as first pointed out by Dr. C. A. White, 

 who also suggested that it should be assigned to a distinct group. (Best. 



