ANTEDON. 407 



surface, and at a later period undergo resorption. When 

 fully developed the larval stalk consists of about twenty- 

 joints, of which the two or three uppermost are short and 

 disc-like (fig. 69). The first whorl of cirri, numbering 

 five, is developed, interradially, upon the under surface 

 of the uppermost joint, which ultimately fuses with the 

 centro-dorsal. The members of the next formed whorl 

 alternate in position with the first, and to these a third 

 whorl is generally added before the young Antedon 

 detaches itself from its stalk and becomes free. The first 

 pinnules are formed before the larva detaches itself from 

 its stalk, and when each of the arms consists of about 

 twelve secundibrachial joints. A bifurcation now presents 

 itself at the growing extremity of the arm, one ramus of 

 which grows more rapidly than the other, and in a line 

 continuous with that of the axis of the arm. The shorter 

 ramus diverges at an acute angle and becomes the primary 

 pinnule. With the exception of the second secundi- 

 brachial, upon which an oral pinnule early appears, the 

 first formed and more proximal joints of the arm do not 

 bear pinnules until the larva become free. 



The larva is capable of considerable movement. Its 

 stalk can be bent from side to side or thrown into a short 

 spiral, while the arms are repeatedly extended and flexed 

 with considerable vivacity. Detachment of the young 

 Antedon from its stalk is not an accidental circumstance, 

 resulting merely from atrophy of the upper joints of the 

 latter. It is preceded by the appearance, at a point 

 immediately below the centro-dorsal plate, of a narrow 

 band of fine fibrils which run parallel to the axis of the 

 stalk, and unite two layers of very minute cells. The 

 plane of rupture passes between these two layers. 

 Detachment does not take place until the cirri are 

 sufficiently developed to enable the animal to attach itself 



