of the Echinoderms. 7 



Instead therefore of the anterior boundary of the fissure 

 of the mouth being formed as in Auricularia by the recurved 

 anterior part of the " ciliated fringe," it is formed by the poste- 

 rior part of a distinct band of cilia. 



It is particularly to be observed that this " band/' like the extra 

 band in the Ophiura-larva, does not encircle the body — it is alto- 

 gether in front of and above the mouth. 



The position of the anus is as in Auricularia. A variety of the 

 Asterid-larva, described by Prof. Muller under the name of Tor- 

 naria, resembles this condition of Bipinnaria, but subsequently 

 adds a ciliated ring like one of those of Auricularia, which encircles 

 the body near the anal end* (fig. 5). 



Bipinnaria increases greatly in size, attaining the length of an 

 inch or more, chiefly by the increase of the anterior part of the 

 body. This assumes a very extraordinary form, both the " band " 

 and the " fringe " throwing out long processes on each side to 

 the number of half a dozen, and at the anterior extremity they 

 form two fin-like expansions placed one above the other. 



Another Asterid-larva, Brachiolaria (Diag. V.), resembles Bi- 

 pinnaria in general form, but developes three processes anteriorly 

 between the anterior part of the ciliated " fringe " and the ante- 

 rior ciliated "band." 



These are all the forms of Echinoderm-larvae enumerated by 

 Prof. Muller. Complicated as they seem to be at first sight, it 

 seems to us that they may all be readily reduced to one very 

 simple hypothetical type ; having an elongated form, traversed by 

 a straight intestine, with the mouth at one extremity and the 

 anus at the other, and girded by a circular ciliated fringe ; just 

 like the larvae of some Annelids (fig. 9). 



Supposing such to be the typical form of the Echinoderm- 

 larva, the specific variations are readily derived from it by simple 

 laws of growth. Let the region before the ciliated fringe be called 

 the pre-trochal region, the region behind the fringe be called the 

 post-trochal region. 



Then the Echinoderm-larvae would appear to be characterized 

 by a disproportionate development of the dorsal post-trochal re- 

 gion (Diag. I a .) whereby the anus is thrust downwards, and the 

 dorsal part of the ciliated fringe downwards and forwards ; pro- 

 cesses are then developed from the ciliated fringe as previously 

 described. 



As in the Annelid-larvae patches of cilia are frequently deve- 

 loped elsewhere than in the principal circle, e. g. on the sides of the 



* If Prof. Mailer's conjecture, that his " wurmformige Larve" (Larven 

 und Metamorphose der Holothurien und Asterien, p. 27) is a further stage 

 of development of Tornaria, be correct, it ultimately assumes a still more 

 worm-like shape, and more closely resembles a Holothurid-larva. 



