1876.] and Development o/Antedon.rosaceus. 217 



ence presents itself in the amount and openness of the " interspace 

 system " to be presently noticed. 



The double wall of the alimentary canal is strengthened by the inter- 

 position of discoidal calcareous plates ; and it is the complex plication 

 of the wall on the central side of the canal, and the consequent piling-up 

 of numerous layers of these disks, which forms the vertical columella, 

 erroneously likened by some writers to the sand-canal of Starfish. 

 Besides these disks, there are occasionally to be found minute calcareous 

 spines projecting from the inner surface into the cavity of the alimentary 

 canal, as if they were pegs whereon to hang the epithelial layer. 



The interior of this columella is traversed by a canal (g, figs. 1, 2), which 

 constitutes a most curious feature in the structure of the visceral mass. 

 "When the visceral mass has been turned out of the calyx, its lower sur- 

 face shows a minute pore nearly in its centre ; which pore, partly occupied 

 (and often concealed) by the pedicle already spoken of (h, figs. 2, 3), is 

 the orifice of communication between the deeper portion of the perivisceral 

 cavity and the " axial canal," which is simply a " survival " of what was 

 originally the part of that cavity segregated from the rest by the winding 

 of the horizontal extension of the alimentary canal round its stomach. 

 The axial canal at first passes upwards almost along the central axis 

 (figs. 2, 3) of the lenticular visceral mass ; but as soon as it meets the 

 obliquely descending commencement of the alimentary canal, it too 

 becomes somewhat oblique, lying in close contiguity to the exterior of 

 the digestive cavity. Before, however, it reaches the oral surface of the 

 visceral mass, it subdivides irregularly into five branches, which embrace 

 the oesophageal part of the alimentary canal, and there become continuous, 

 as I have already mentioned, with the five radial canals that lie beneath the 

 tentacular canals of the oral disk, one of which is seen in transverse 

 section at I, fig. 1, and others at i, Jc. Thus, through the subdivision of 

 the five radial canals of the disk into the ten brachial canals, each of 

 which gives off lateral branches that proceed to the extremities of the 

 pinnules, there is a continuous canal-communication from the axial canal, 

 and hence from the deeper part of the perivisceral cavity through the 

 entire apparatus of arms and pinnules. But the axial canal is also in 

 relation with the wall of the alimentary canal ; for between the plications 

 of the inner or central side of the latter there is a very extensive system 

 of irregular passages or interspaces (&, fig. 3), which open into the axial 

 canal ; and it can scarcely be doubted that these serve, like the mesenteric 

 vessels of higher animals, to receive the nutritive fluid which has trans- 

 uded through the wall of the alimentary canal, first through the mucous 

 lining into the intramural space, and thence through its peritoneal cover- 

 ing into the perivisceral cavity and its derivations. 



Cavity of the Centro-dorsal Basin. — In my previous Memoir I fully 

 described the " centro-dorsal" basin-shaped plate whereon rests the 

 circlet of first Eadials, whilst from its free or dorsal surface extend the 



