414 



PELMATOZOA. 



Though free in its adult condition, the Rosy Feather-star passes 

 through a stage of its development in which it is attached by a 

 delicate jointed stalk to some foreign object (fig. 288). When 

 first discovered in this condition, it was supposed to be a distinct 

 type of the Crinoids, and was described under the name of Pe?tta- 

 crinus Europeans. The Comatula, therefore, represents temporarily, 

 in this stage of its development, the permanent condition of the 

 Pedunculate Crinoids. 



As regards the development of Comatula, the larva is at first cylindri- 

 cal, with four transverse bands of cilia, a hinder tuft of cilia, and an ali- 

 mentary canal furnished with a lateral aperture, its general aspect closely 

 resembling that of the embryos of certain Annelides. The skeleton of the 

 calyx is developed anteriorly, that of the column 

 posteriorly, the former being the first to appear. 

 In its early condition (fig. 288) the calycine skele- 

 ton consists of a row of five " basal " plates (6), 

 together with three or five small " underbasals," 

 which rest below upon the so-called " centrodor- 

 sal plate " (cd), and fuse with it at an early period. 

 The basals are succeeded above by a cycle of 

 five " oral " plates (o), in the centre of which the 

 permanent mouth is finally developed. Five 

 " radial " plates (r) are next developed as a cycle 

 between the oral and basal plates : and to the 

 radials are rapidly added the plates of the arms 

 proper (the " brachials). Inferiorly, the centro- 

 dorsal plate rests upon a short jointed column 

 (fig. 288, c), at the bottom of which is an ex- 

 panded " dorsocentral" plate (d), forming 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 

 centrodorsal plate increases in size, so as to en- 

 close the basal plates, which in turn become 

 fused with one another, and remain only as the 

 so-called " rosette " on the upper surface of the 

 centrodorsal. The latter also develops jointed 

 cirri from its outer suface, and finally becomes 

 detached from the next joint of the column be- 

 low, when the animal enters upon its free stage of 

 life. 



Fig. 288. — Larva of Coma- 

 tula (Antedon) rosacea, en- 

 larged (after Sir Wyville 

 Thomson), o o, Oral plates ; 

 r r, Radial plates ; b b, Basal 

 plates ; cd, Centrodorsal plate ; 

 c, Column ; d, Disc of attach- 

 ment (dorsocentral plate). 



As regards the essential features in their 

 anatomy, the " Stalked " Crinoids do not 

 differ materially from the " Free " forms of the group. More 

 particularly, there is a substantial identity in structure in the two 

 sections of the order as regards the form and arrangement of the 

 alimentary canal, the ambulacral and vascular systems, and the ner- 

 vous and reproductive organs. As the majority of the fossil Crin- 



