of the Ecbinoclerms. 7 



Instead therefore of the anterior boundary of the lissure 

 of the mouth being formed as in Auricularia by the recurved 

 anterior part of the " cihated fringe/' it is formed by the poste- 

 rior part of a distinct band of cilia. 



It is ])articularly to be observed that this " band/' hke 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 Auriailaria. A variety of the 

 Asterid-larva, described by Prof. jNliiller under the name of Toi-- 

 naria, resembles this condition of Bipinnana, but subsequently 

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

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



Bipinnaria increases greatly hi size^ attaining the length of an 

 inch or more, chielly 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 tin-like expansions placed one above the other. 



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

 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. Miiller. 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 larva of some Annelids (tig. 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 cihated fringe be called 

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

 post-trochal region. 



Then the Echinoderm-larvai would appear to be characterized 

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

 gion (Diag. !•'.) 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-larvse patches of cilia are frequently deve- 

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



* If Prof. Miiller's conjecture, that his " wurmfdrmige Larve " (Larven 

 \\m\ Metamorphose <ler Ilolothuriea und Asterien, j). 27) is a further stage 

 of «k'velo|)incnt of Toniurifi, be correct, it ultimately assumes a still more 

 ^vonn-likc sliaiH', and more closely resembles a Ilolothurid-lai-va. 



