36 



calyx, supported, as we have already seen, by the circlets 

 of basal and oral plates. The lowest joint of the 

 stem (figs. GH, 67, and 69) assumes a discoid or lobate 

 form, so as to afford firm attachment to the substratum 

 upon which the larva has fixed itself, whilst the highest is 

 also somewhat expanded as a support for the calyx. In 

 the latter the basal and oral plates are now fully 

 developed. 



As will be seen in fig. (i6, the oral plates usually stand 

 almost erect, thus exposing the underlying oral apparatus, 

 but they can be closed down upon it. Five pairs of 

 tentacles make their appearance between the groups of 

 three primary ones (fig. 68, se.ten.), making twenty-five 

 in all; and whereas at first the five tentacles in each 

 radius are connected by a common tentacle canal arising 

 from the circum-oral water vessel, they eventually arise 

 separately from the latter. Another circlet of plates, the 

 radials, now make their appearance in the spaces left 

 between the contiguous angles of the basal and oral plates 

 (fig. 67, 69, and 70, rd.). As their name implies, they 

 are radial in position. A small asymmetrical plate, the 

 anal, also appears between two of the radials, and on a 

 level with them, but this undergoes resorption at a later 

 period. The next plates to appear are the first primi- 

 brachials, which, supported by the radials, project upwards 

 and slightly outwards between the orals (fig. 67, j^mh. 1). 

 These are soon followed by the second primibrachials or 

 axillaries {pmh. 2), upon the distal extremities of which 

 the first pair of secundibrachials [sec. 1) appear in due 

 course. Eventually the basal plates fuse to form the 

 rosette, which covers in the chambered organ and central 

 capsule. By the outward growth of the arms and the 

 consequent enlargement of the circumference of the 

 tegmen calycis, the oral plates are left upon the oral 



