CLASS III CRINOIDEA 181 



All of the calcareous elements of which the dorsal skeletal system is com- 

 posed are developed within a uniform organic base ; with the growth of the 

 plates this within them becomes a diffuse network, and between them forms 

 a mass of strong connecting fibrils which bind the plates together. 



In the so-called sutures between the calyx plates, in the intercolumnar 

 articulations in many of the older types (Encriiius, etc.), in the syzygies between 

 the brachials in many forms, and just below the cirrus-bearing joints in the 

 stems of the Pentacrinites, these fibrils are all of uniform length and uniformly 

 disti'ibuted over the joint face ; but usually there is a differentiation of the 

 fibrils by which they become more or less segregated into radial groups, as in 

 the stems of Pentacrinites ; or they become elongated and differentiated into two 

 comparatively dense masses separated by a strong fulcral ridge, and assume a 

 more or less contractile function as in the stems of such genera as Bhizocrinus 

 and Platycriniis, in the cirri of the Pentacrinites and Comatulids, and in the 

 pinnules beyond the second joint in many forms. Between the brachials they 

 are usually differentiated into two distinct types, one of which, occupying the 

 entire dorsal half of the joint face (the dorsal ligament) is comparable to one 

 of the two masses in the type just described ; while the other, occupying two 

 more or less triangular areas, one on either side of the central canal and just 

 ventral to the transverse ridge by which they are separated from the preced- 

 ing (the interarticular ligaments), is much more dense, and serves to bind the 

 brachials tightly together. 



In addition to these ligamentous connections there are, between the 

 brachials, two muscular bundles situated on the ventral border of the joint 

 face, distal (ventral) to the interarticular ligament masses. Whereas the 

 ligament bundles are developed directly from the original uniform body 

 investment in which the calcareous elements are formed, the muscular bundles 

 have an entirely separate origin. 



In certain of the older forms the proximal segments of the column 

 occasionally exhibit simple vertical clefts, which indicate an original quin- 

 quipartite composition. These divisions always occur alternately with those 

 of the basals in monocyclic, and with those of the infrabasals in dicyclic 

 forms. The entire crinoid stem is probably the homologue of a single apical 

 calyx plate, which has been reduplicated by a curious process of serial repeti- 

 tion common among the Echinoderms. 



Ontogeny. — Although we are as yet acquainted with the life-history of but 

 thi'ee species, all belonging to the genus Antedon, and although the life- 

 history of but two of these {A. mediterranea and A. adriatica) is well under- 

 stood, the phenomena of their development are of such significance as 

 to shed most valuable light upon many conditions observed in fossil 

 Crinoids. 



The eggs, extruded from the ovaries and hanging in little groups from the 

 genital pinnules, are fertilized externally, and the early metamorphosis of the 

 larva takes place within the egg membrane. At the time of the rupture of 

 the egg membrane and its consequent escape the embryo (a gastrula) is elongate- 

 oval in form, bilaterally symmetrical, bearing an anterior tuft of cilia and 

 encircled by five ciliated bands, resembling somewhat the larvae of certain 

 annelids. Internally there are to be seen the rudiments of five oral plates, 

 five basals, three (A. mediterranea) or five {A. adriatica) infrabasals, and about 

 eleven columnars ; the orals, basals, and infrabasals are arranged in horseshoe- 



