ORAL AND APICAL SYSTEMS OF THE ECHINODERMS. 353 



comparative development of these two systems in the other 

 Echinoderms. 



The first observer who paid any attention to the homo- 

 logies of the skeletal plates of the young Crinoid was Pro- 

 fessor Allman. He described the body of the larva as 

 consisting of a calyx covered by a pyramidal roofJ 



" The calyx is composed chiefly of five large plates, very 

 distinct, and united to one another by simple suture (fig. 

 I, 3, 3). Between the lower edges of these plates and the 

 summit of the stem is a narrow zone {cd), in which no 

 distinct indications of a composition out of separate plates 

 can be detected. Between the upper angles of every two 

 contiguous large plates there may, with some care, be made 

 out a minute intercalated plate (4,4). There would thus 

 be five of these little intercalated plates, which, though by 

 no means so evident as the large plates which alternate with 

 them, are sufficiently so to leave no doubt of their presence. 



The pyramidal roof which closes the cup in the 



contracted state of the animal is composed of five large tri- 

 angular plates (or), each supported by its base upon the 

 upper edge of one of the large plates of the calyx, and with 

 the small intercalated plates encroaching upon its basal 

 angles." 



Allman recognised at once that the narrow zone {cd), 

 intervening between the five large hexagonal plates and the 

 summit of the stem, represented the centrodorsal piece of 

 the adult Comatula, and he regarded it, in complete ac- 

 cordance with Miiller's views, as a metamorphosed stem- 

 joint. The five large plates (3, 3) superimposed upon it and 

 constituting almost the whole of the calyx, were regarded by 

 him^ as corresponding " to the true Basalia which im- 

 mediately surround the stem in such forms as Platycri7ius, 

 and the great majority of the Crinoidea; while the five 

 small plates (4, 4) intercalated between their upper angles 

 will represent radialia.''^ 



While believing this view of the homologies of the 

 elements of the calyx in the Pentacrinoid to be the correct 

 one, Allman suggested another as possible, which has not 

 found general acceptance, though it has recently been revived 

 by Agassiz and Loven. 



It is as follows. — The single centrodorsal jnece {cd) 

 represents a zone of coalesced Basalia, the " Pelvis " 



^ " On a Pre-bracbial Stage iu the Development of Comatula, aud its 

 importance iu its Relation to certain aberrant forms of Extinct Crinoids," 

 •Transactions of the Royal Society of Edinburgh,' vol.xxiii, p. 241. 



' Loo. cit., p. 244. 



