476 



GRINOID LARVA. 



bilateral, owing to a flattening of the ventral surface. On the 

 flattened surface appears a ciliated depression corresponding in 

 position with the now closed blastopore (vide p. 459). The third 

 ciliated band bends forward to pass in front of this (fig. 269). Be- 

 hind the last ciliated band there is present a small depression of 

 unknown function, also situated on the ventral surface. The posterior 

 extremity of the embryo elongates to form the rudiment of the future 

 stem, and a fresh 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 

 (fig. 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 trans- 

 verse, but form two oblique circles, 

 the dorsal end being in advance of 

 the ventral. The rows soon become 

 transverse, while the originally some- 

 what 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 symmetry. While the first 

 skeletal elements of the calyx are 

 being formed, the skeleton of the 

 stem is also established. The ter- 

 minal plate is first of all established, 

 then the joints, eight at first, of the 

 stem. The centro-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 established 

 the ciliated bands undergo atrophy, and shortly afterwards 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 



1 Gotte (No. 549) on the other hand holds that the centro-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 hasals, and that it is therefore 

 similar in its origin to the basals. 



FIG. 269. LARVA OF ANTEDON WITH 

 RUDIMENTS OF CALCAREOUS SKELETON. 

 (From Carpenter; after Thomson.) 



1. Terminal plate at the end of 

 the stem; 3. basals; or. orals; II. 

 position of blastopore. 



