502 THE ANATOMY OF IXVEETEBRATED ANIMALS. 



first asserted by Ludwig, 1 cannot be doubted. Immediately 

 beneath it runs a small canal, discovered by Dr. Carpenter, 

 and termed by him the tentacular canal, which gives off lat- 

 eral branches to communicate with the cavities of the pedi- 

 cels. A second much wider canal — the subtentacular canal — 

 lies beneath this, and is divided by a longitudinal septum. 

 But the septum is incomplete at intervals, and thus the two 

 canals communicate. A third, still larger — cceliac canal — is 

 interposed between the floor of the subtentacular canal and 

 the axial skeleton of the arm. 



Where the arm joins the calyx the tentacular canals run 

 beneath the ambulacral groove to the gullet, around which 

 they are united by a circular canal, from which numerous 

 short diverticula, resembling the vasa ambulacralia cavi in 

 the Ophiurids, described by Simrock {I. c), depend. The 

 subtentacular and cceliac canals communicate with channels 

 in the perivisceral tissue, on the oral or the aboral face of the 

 visceral mass ; and these channels appear, eventually, to 

 open freely into the cavities by w T hich the columella is trav- 

 ersed. 



In the partition between the subtentacular and the cceliac 

 canals there lies a cellular cord, or rachis, which can be traced 

 back into a reticulation of similar tissue in the visceral mass. 

 The genital glands contained in the pinnules are enlargements 

 of lateral branches of this rachis. But the rachis is appar- 

 entlv only an extension of the mesodermal tissue of the vis- 

 ceral mass, comparable to that in which the genitalia are 

 lodged in the Star-fishes ; and the multiplication of the geni- 

 tal glands may be regarded as a further extension of the 

 structure which obtains in Brislnga. Thus it would seem 

 that the position of the genital glands in the Crinoids is not 

 so anomalous as it at first appears to be. 



The centro-dorsal tubercle contains a cavity with which 

 the canals which traverse the ossicula of the cirri, the calyx, 

 the brachia, and the pinnules communicate. This cavity was 

 considered by Mttller to be a heart. It proves, however, to 

 be largely filled by solid tissue, which is continued not only 

 into all the canals which traverse the ossicula, but also into 

 the columella, or tissue which occupies the centre of the coils 

 of the alimentary canal. 



Dr. Carpenter 2 is of opinion that so much of this axial 



1 Zeiischrift fur wiss. Zoologie* 1876. 



2 " Proceedings of the Royal Society," 1876. 



