V1I[ 



EGHINODEBMA TA— ONTOGENY 



501 



in consequence a more complicated form, and takes a winding coui'se. This descrip- 

 tion will be elucidated by the figures. 



Here, as in all other Echinoderms, the ciliated rings are mere remains of the 

 cilia which covered the whole body in an earlier stage, i. e. in the gastrula. 



2. Asteroidea (Fig. 398, B). — The Astei'oid larvce are known as Bipinnarise and 

 Brachiolarise. The chief distinction between them and the Auricularia is the 

 preoral ciliated ring. This is a circle on the preoral area, and within the circumoral 

 ciliated ring, from which it is altogether distinct. 



The comparison of a Bipinnaria with an Auricularia led to the conjecture that 

 the preoral ciliated ring of the former corresponds with a preoral portion of the 

 common circumoral ciliated ring of the latter, which has become distinct and has 

 closed to form a ring. Direct observation of the ontogenetic development of the 

 ciliated ring of the Asteroid larva has entirely confirmed this conjecture. 



B 



Pig. 398.— a, B, C, Auricularia, Bipinnaria, and Tornaria (Enteropneustan larva), from the 

 right .side, diagrammatic. 1, Pretroclial area ; 2, oral area ; 3, postoral area ; 4, anal area ; I, pre- 

 oral ; II, circumoral : III, anal or principal ciliated ring ; 5, neural plate ; os, mouth, an, anus. 



The Bipinnaria passes through an Auricularia stage. The general ciliation of 

 the body, belonging to an early stage, disappears first from the ventral side, which 

 becomes depressed, then from the dorsal side, in such a way as to leave a band 

 running back on itself at the edge of the ventral depression ; this corresponds entirely 

 with the course of the circumoral ciliated band in the Atiricularia. In the frontal 

 region, however, where the two lateral strips of the circumoral band approach each 

 other in the median line, a ciliated island is for a time retained connecting them 

 {Asterias rubens). The covering of cilia thus forms an X-like cross on the frontal 

 region. By the disappearance of the ciliation from the centre of the X, the preoral 

 section of the ciliated ring is separated from the rest, and forms the distinct prebral 

 ring enclosed within the circumoral ring. 



The process in Asterias vulgaris seems to take a somewhat different course, but 

 has the same final result. On the frontal region, where, in A. rubens, an isolated 

 ciliated area remained to form a connection between two portions of the circumoral 

 ciliated band, this connection arises only secondarily by the approximation of the 

 two portions in the middle line. The further process of separation of the preoral 

 ring from the rest, which latter then represents the secondary circumoral ciliated 

 ring, agrees with that in A. rubeus. 



The ventral depression (in which the mouth lies) which, in the Auricularia, runs 

 forward to the right and left of the preoral portion of the circumoral ciliated ring. 



