30 LAMELLIBRANCHIA. 



early finds expression in the rudiment of the mesoderm and in the 

 position of the blastopore. 



3. Development and Structure of the Trochophore Larva. 



There is, in the development of the Lamellibranchia, a stage 

 which more or less closely resembles the Trochophore larva of the 

 Annelida, and which has therefore received the same name (Ray 

 Lankester, Hatschek). This stage is most marked, as we should 

 naturally expect, when it is represented by a free-swimming larva, 

 such as is found among the marine Lamellibranchs [Teredo, Car- 

 dium, Mijtilux, Ostrea, etc.), but can be clearly recognised also in 

 other forms (Cyclas, Pisidinm). In the Unionnlap, the Trocliophore 

 stage has undergone much greater modification. Thus among the 

 marine Lamellibranchs we find, as a rule, that the primitive larval 

 form has been retained in a less specialised condition than among the 

 fresh-water forms, and this affords a further confirmation of a pheno- 

 menon which is very wide-spread in the animal kingdom. One fresh- 

 water Lamellibranch, however, Dreissemia polymorpha (evidently in 

 consequence of its late transference to fresh water) exhibits a larva 

 agreeing exactly with those of the marine Lamellibranchia (Kor- 

 schelt, No. 27, .Blochmann, No. 3, Weltner, No. 58). 



The structure and development of the Trochophore larva have been 

 best investigated by Hatschek in Teredo ; in addition, Brooks and 

 Horst have published observations upon the larva of the Oyster, 

 and Loven upon those of various other Lamellibranchs (Modiolaria, 

 Cardium, Montacuta). The Trochophore stage of the fresh-water 

 Lamellibranchs has been carefully investigated in Ct/clas by Ziegler. 

 We shall here for the most part follow Hatschek' s account of the 

 larva of Teredo, since this form, of all those as yet known, most 

 clearly exhibits the Trochophoran condition. The larva of Ostrea 

 edulis which, with regard to the formation of the alimentary canal, 

 shows (according to Horst) a still simpler condition, agrees very 

 closely with Teredo. 



A. The Trochophore stage as a free-swimming larva. 



We have already (p. 25) described a few stages in the development 

 of Teredo, in which an epibolic gastrula is formed (Fig. 12 A-O). 

 Further changes begin by the overgrowth of the mesoderm-cells lying 

 at the edge of the blastopore by the ectoderm ; the former thus 

 become enclosed within the embryo, the blastopore closing in con- 



