THE CRINOIDEA. 501 



another and to the centro-dorsal plate, and are not visible on 

 the outer surface of the calyx. The space left between the 

 apices of the five first radials is occupied by a single plate, 

 the rosette, 1 which is formed by the coalescence of the five 

 basalla present in the larva. 



The anatomy of the soft parts of the Crinoidea has been 

 most thoroughly investigated in the genus Comatula (An- 

 tedon).' 1 



The mouth leads, by a short, wide gullet, into a spacious 

 sacculated alimentary canal, which is coiled upon itself in 

 such a manner as to make about one turn and a half around 

 the axis of the body, and then terminates in the projecting 

 rectal cone, which, as has already been seen, is situated inter- 

 radially on the oral face of the calyx. The central cavity, 

 included by the coil of the alimentary canal, is occupied by a 

 sort of core of connective tissue, and has received the name 

 of columella, but it must be understood that it is not a dis- 

 tinct structure. Bands of connective tissue connect the outer 

 periphery of the alimentary canal with the perisoma. 



The five triangular lobes of the perisoma, which surround 

 the mouth like so many valves, contain no calcareous skele- 

 ton in the adult Antedon. Within these lobes, attached to 

 the oral membrane, there is a circle of tentacula. From the 

 interval between each pair of oral valves, a groove radiates 

 outward over the surface of the calycine perisoma and speed- 

 ily bifurcates ; one branch goes to the oral surface of each of 

 the arms, and runs along it to its extremity, giving off alter- 

 nate lateral branches to the pinnules in its course. 



These grooves are the ambulacral grooves. Their sides 

 are, as it were, fenced by small, lobed processes of the peri- 

 soma ; and, on the inner sides of these processes, groups of 

 minute pedicels take their origin from the sides of the floor 

 of the groove. A thickened band of the ectoderm occupies 

 the middle of the floor, and so strikingly resembles the ambu- 

 lacral nerve of the Star-fish that the homology of the two, 



1 Carpenter, " On the Structure, Physiology, and Development of Comatula." 

 ("Phil. Trans.," 1866.) 



2 E. Perrier, " Eecherches sur l'Anatomie de la Comatula rosacea" ("Arch. 

 de Zoologie Experimentale," 1873). Semper, " Kurze anatomische Bemerkun- 

 gen uber Comatula" (" Wiirzburg Arbeiten," 1874). Ludwig, " Zur Anatomie 

 der Orinoideen" (Zeitscliriftfiirwiss. Zool., 1876). Carpenter, "On the Struct- 

 ure, Physiology, and Development of Antedon" ("Proc. Eoval Society," 

 1876). Greef, " Ueber den Bau der Crinoideen " (" Marburg Sitzuhgsberichte," 

 1876). P. H. Carpenter, " Ptemarks on the Anatomy of the Arms of the Cri- 

 noids" {Journal of Anat. and Physiology, 1876). 



