ANATOMY AND DEVELOPMENT OF THE ECHINODERMS IO9 



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 posterior part of a distinct band 

 of ciHa. 



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

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

 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 Miiller under the name of Tornaria^ 

 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, Bi'achiolaria (Diag. V. [Plate 1 2]), resembles 

 Bipinnaria in general form, but developes three processes anteriorly 

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

 ciliated " band." 



These are all the forms of Echinoderm-larvje 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 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 region (Diag. 

 P. [Plate 12]) whereby the anus is thrust downwards, and the dorsal 

 part of the ciliated fringe downwards and forwards ; processes are 

 then developed from the ciliated fringe as previously described. 



^ If Prof. Miiller's conjecture, that his " vvurmfcirmige Larve " (Larven und Metamor- 

 phose der Holothurien und Asterien, p. 27) is a further stage of development of Totiiaria, 

 be correct, it ultimately assumes a still more worm-like shape, and more closely resembles a 

 Holothurid-larva. 



