TYPE ECHINODERMA. 



551 



appear in the Upper Silurian and also disappear in the Carboniferous. 

 For a description of these forms reference must be made to the standard 

 works on Palaeontology. On account of their similarity to the Crinoids they 

 have been associated with them in the class Pelmatozoa, of which each of 

 the groups formed an order; inasmuch, however, as the present work is 

 concerned only with recent forms ; it has been thought more convenient to 

 regard the Crinoids as a class. 



The class Crinoidea has been divided into two orders. The Palceocri- 

 nida, chiefly Palaeozoic forms, characterized principally by the presence of 

 under-basals and of a series of plates covering in the disk almost com- 

 pletely, to which may be added the usual presence of interradials and the 

 greater width of one of the interradii, that in which the anus occurred 

 (see Fig. 250). The Neocrinida, on the other hand, included the recent 

 forms, the group making its first appearance in the Mesozoic, and is char- 

 acterized by the disk being only imperfectly covered by plates, by the 

 under-basals and interradials being absent, and the interradii all equal in 

 width. Transition forms between the two groups occur, however, the 

 genera Hyocrinus and Calamocrinus, for example, presenting certain 

 Palteocrinid peculiarities combined with Neocrinid ones, and it seems more 

 satisfactory to divide the class into families only, leaving orders out of the 

 question. 



Development of the Crinoids. Antedon is the only Criuoid whose 

 development has been studied. The embryo leads 

 for a time a free-swimming existence, and possesses 

 a somewhat ovoidal form (Fig. 253) with a tuft of 

 cilia at the smaller anterior end and five rings of 

 cilia surrounding the body. Not far from the ante- 

 rior end is a slight groove, and lower down upon the 

 side is a much larger one. This larva settles down 

 upon the anterior end, the slight depression near 

 this end serving as an organ of fixation, and then 

 a rather remarkable rotation occurs, the large groove 

 shifting round together with the interior organs until 

 it comes to lie at the free end of the organism, and at 

 the same time its lips unite so as to enclose a cavity, 

 the vestibule. Calcareous plates have ere this de- 

 veloped in the connective tissue of the embryo and 

 outline a stalked Crinoid into which the larva is 

 gradually transformed, the larval skin shrinking as 

 it were, so as to closely surround the stalk and calyx, 

 while the vestibule opens to the exterior by the 

 gradual thinning and final disappearance of its roof, 



its floor forming the ectoderm of the disk. After continuing its growth for 

 some time as a stalked Crinoid, the young Antedon finally separates from 

 its stalk, and thereafter leads a free existence. 



FIG. 253. LARVA OF 



Antedon (combination 

 of figures by THOMPSON 

 AND GOETHE after KOR- 



SCHELT AND HEIDER). 



