1876.] and Development of Antedon rosaceus. 229 



forming the coeliac canal (cc, fig. 12) of the rays and arms, with the ven- 

 tral division of the coelom always remains freely open ; but that of the 

 upper or oral extension forming the subtentacular canals (stc) of the 

 rays and arms, with the oral division of the coelom itself, is gradually 

 limited by the enlargement of the visceral mass, and by the formation of 

 adhesions between its upper surface and the under surface of the oral 

 disk; so that, as the vertical communication between the upper and 

 lower divisions of the coelom becomes gradually narrowed into the " axial 

 canal," so the oral portion of the coelom may be said to become gradually 

 narrowed into the subtentacular canals of the disk. 



The principal changes of note which occur in the further development 

 of the visceral mass consist in the progressive infolding of the wall of 

 the alimentary canal on the central side of the tube, so as to form the 

 gland-like projection into its cavity ; the development of the calcareous 

 disks in the inner or mucous layer of that wall, the piling-up of which in 

 its infolded portions constitutes the " columella ;" the separation of the 

 outer or peritoneal layer from the mucous, forming a space which is 

 crossed by multitudinous threads and lamellae of connective tissue ; the 

 narrowing of the spaces formed by the infolding of the walls of the ali- 

 mentary tube into the mesenteric canals which open into the axial canal ; 

 and, generally, the development, in various parts, of a large quantity of 

 connective tissue, which traverses what would otherwise be vacuities, 

 and gives compactness to the whole "visceral mass." 



The mode of development of the peripheral portion of the summit of 

 the original Crinoidal axis into the quinquelocular organ Contained within 

 the centro-dorsal plate has been already noticed ; and as I described in 

 my former Memoir the very curious manner in which its radial cords, 

 which at first lie on the upper or ventral side of the basal plates, finally 

 come to lie on the lower or dorsal side of the "rosette" formed by their 

 metamorphosis, I need not here repeat the history. 



EXPLANATION OF THE PLATES. 



Plate 8. 



Fig. 1. Horizontal Section of the Visceral mass of Antedon rosaceus (after decalcification 

 of the skeleton), laying open the intestinal coil : — a, termination of the 

 oblique or oesophageal portion in the horizontal or intestinal portion b, c, 

 which forms a cul-de-sac at d, where it abruptly bends upwards to form the 

 projecting vent; at^is seen the axial canal, surrounded by the columella 

 formed by the infolding and duplication of the intestinal wall ; f, con- 

 nective tissue and interspace-system, shown at e to be an extension of the 

 space between the two lamellas of the intestinal wall. 



Fig. 2. Vertical Section of the Calyx : — a, mouth, surrounded by oral ring, and sur- 

 mounted by the two tentaculiferous lobes mm; at b is seen the oblique or oeso- 

 phageal portion of the alimentary canal, the horizontal or intestinal coil of 

 which is transversely divided at c and d, showing in section the corrugations 

 of its double wall, the internal surface of which is covered by a thick 

 (hepatic?) epithelial layer, e,f. At g is seen the lower part of the axial 



