ECHINODERMATA : CRINOIDEA. 



215 



selves into a central group (of one or more) and five peripheral vessels, the 

 latter of which expand in the calyx into the five chambers of the "cham- 

 bered organ," the chambers and axis alike giving branches to the dorsal 

 cirri. In the pedunculate Crinoids in which there are no cirri, the 

 chambers narrow again, and the group of vessels is continued down the 

 central canal of the column. In Pentacrinus, which has cirri at regular 

 intervals, the five peripheral vessels expand in each cirrus-bearing joint into 

 five dilatations, which thus give rise to a miniature "chambered organ," 

 each chamber of which gives off a single vessel to a cirrus. In the body, 

 the vascular axis is connected with (i) a large network of vessels round the 

 alimentary canal, (2) an extensive plexus beneath the ventral surface of the 

 disc, in which vessels arise that run out into the arms and enclose the gen- 

 ital glands, and (3) a plexus of convoluted tubes depending from the oral 

 blood-vascular ring in which the radial vessels of the arms originate (P. H. 

 Carpenter). 



The reproductive organs of Comatula are situated beneath 

 the soft skin of the arms, and their ducts open into the pinnulae, 

 by the rupture of the ventral integument 

 of which the generative elements are 

 set free into the surrounding water. 



As regards the development of Comatula, the 

 larva is at first cylindrical, with four transverse 

 bands of cilia, a hinder tuft of cilia, and an 

 alimentary canal furnished with a lateral aper- 

 ture, its general aspect closely resembling that of 

 the embryos of certain Annelides. The skele- 

 ton of the calyx is developed anteriorly, that of 

 the column posteriorly, the former being the 

 first to appear. In its early condition (fig. 106) 

 the calycine skeleton consists of a row of five 

 "basal" plates, which rest below upon the 

 so-called "centro-dorsal plate," and are suc- 

 ceeded above by a cycle of five "oral" plates, in 

 the centre of which the permanent mouth is fin- 

 ally developed. Five " radial " plates are next 

 developed as a cycle between the oral and basal 

 plates ; and to the radialsare rapidly added the 

 plates of the arms proper (the "brachial" 

 plates). Inferiorly, the centro-dorsal plate rests 

 upon a short, jointed column (fig. 106, c\ the 

 lowest plate of which is expanded to form a 

 disc of attachment ; and the larva now passes 

 into what is known as its " Pentacrinus stage." 

 In the further progress of growth the arms 

 increase in length, and the oral plates diminish 

 in size and ultimately disappear. At the same 

 time the centro-dorsal plate increases in size, 

 so as to enclose the radial plates, which in turn 

 become fused with one another, and remain 

 only as the so-called "rosette" on the upper 

 surface of the centro-dorsal. The latter also 

 develops jointed cirri from its outer surface, and 

 finally becomes detached from the next joint of the column below, when 

 the animal enters upon its free stage of life. 



Fig. 106. Larva of Comatula 

 (Antedori) rosacea, enlarged 

 (after Sir Wyville Thomson). 

 o o Oral plates ; r r Radial 

 plates ; b b Basal plates ; c d 

 Centro-dorsal plate ; c Col- 

 umn ; d Disc of attachment. 



