I . MINODKKMATA. 



571 



situated on the ventral surface. The posterior extremity of the 

 embryo elongates to form the rudiment of the future stem, and 

 >h depression, marking the position of the future mouth, 

 makes its appearance on the anterior and ventral part. 



While the ciliated bands are still at their full development, 

 the calcareous skeleton of the future calyx makes its appearance 

 in the form of two rows, each of five plates, formed of a network 

 of spicula (figs. 268 B and 269). The plates of the anterior ring 

 are known as the orals, those of the posterior as the basals. 

 The former surround the left, i.e. anterior 

 peritoneal sack ; the latter the right, i.e. 

 posterior peritoneal sack. The two rows 

 of plates are at first not quite transverse, 

 but form two oblique circles, the dorsal 

 end being in advance of the ventral. 

 The rows soon become transverse, while 

 the originally somewhat ventral oral 

 surface is carried into the centre of the 

 area enclosed by the oral plates. 



By the change in position of the 

 original ventral surface relatively to the 

 axis of the body, the bilateral symmetry 

 of the larva passes into a radial sym- 

 metry. While the first skeletal elements 

 of the calyx are being formed, the 

 skeleton of the stem is also established. 

 The terminal plate is first of all esta- 

 blished, then the joints, eight at first, of 

 the stem. The ccntro-dorsal plate is 

 stated by Thomson to be formed as the 

 uppermost joint of the stem 1 . The larva, after the completion 

 of the above changes, is shewn in fig. 268 B, and somewhat more 

 diagrammatically in fig. 269. 



After the above elements of the skeleton have become es- 

 tablished the ciliated bands undergo atrophy, and shortly after- 



: tc (No. 549) on the other hand holds that the ccntro-dorsal plate is developed 

 by the coalescence of a series of at first independent rods, which originate simul- 

 taneously with, and close to, the lower edges of the basals, and that it is therefore 

 similar in its origin to the basals. 



Fio. 169. LARVA or 

 ANTEDON WITH RUDIMENTS 

 OP CALCAREOUS SKELETON. 

 (From Carpenter; after 

 Thomson.) 



i. Terminal plate at the 

 end of the stem ; 3. basals ; 

 or. orals; bl. position of Mas- 

 toporc. 



