572 



CRINOID LARVA. 



wards the larva becomes attached by the terminal plate of its 

 stem. It then passes into the Pentacrinoid stage. The larva in 

 this stage is shewn in fig. 268 C and fig. 270. New joints are 

 added at the upper end of the stem next the calyx, and a new 

 element the radials makes its appearance as a ring of five 

 small plates, placed in the space between the basals and orals, 

 and in the intervals alternating with them 

 (fig. 270, 4). The roof of the oral vesti- 

 bule (vide fig. 253 and p. 551) has in 

 the meantime become ruptured ; and 

 the external opening of the mouth thus 

 becomes established. Surrounding the 

 mouth are five petal-like lobes, each of 

 them supported by an oral plate (fig. 

 268 C). In the intervals between them 

 five branched and highly contractile ten- 

 tacles, which were previously enclosed 

 within the vestibule, now sprout out : 

 they mark the position of the future 

 radial canals, and are outgrowths of the 

 water-vascular ring. At the base of each 

 of them a pair of additional tentacles is 

 soon formed. Each primary tentacle cor- 

 responds to one of the radials. These 

 latter are therefore, as their name implies, 

 radial in position; while the basals and 

 orals are interradial. In addition to the 

 contractile radial tentacles ten non-con- 

 tractile tentacles, also diverticula of the 

 water- vascular ring, are soon formed, two 

 for each interradius. 



In the course of the further develop- 

 ment the equatorial space between the 

 orals and the basals enlarges, and gives 

 rise to a wide oral disc, the sides of which 

 are formed by the radials resting on the 

 basals ; while in the centre of it are 

 placed the five orals, each with its special lobe. 



The anus, which is formed on the ventral side in the position 



FIG. 270. YOUNG PEN- 

 TACRINOID LARVA OF AN- 

 TEDON. (From Carpenter ; 

 after Wyville Thomson.) 



i . terminal plate of stem ; 

 cd. centro-dorsal plate ; 3 . 

 basals; 4. radials; or. orals. 



