THE TROCHOPHORE STAGE AS A FREE-SWIMMINQ LARVA. 



31 



Y.m. 



sequence of the further growth of the ectoderm (Fig. 12 C). The 

 relation of the blastopore to the mouth which now forms could not 

 be established in Teredo, but the closure of the blastopore on the 

 ventral sick' seems to take place in the neighbourhood of the future 

 mouth. This latter arises at a somewhat later stage in the form of 

 an ectodermal invagination (Fig. 15 A). A comparison of this stage 

 with those that lead to the Trochophore shows us that the longitu- 

 dinal axis of the latter 

 is not identical with 

 that of the gastrula, 

 but apparently cuts 

 it at almost a right 

 angle. A similar con- 

 dition is found in 

 Ostrea (Fig. 14 A-E). 

 In the Oyster, the 

 blastopore does not 

 close, but becomes 

 carried into the in- 

 terior of the embryo 

 by an invagination of 

 the ectoderm, the 

 stomodaeum. The 

 blastopore thus per- 

 sists as the opening 



between the stomodaeum and the enteron. The transformation 

 of the archenteron into the enteron of the larva can take place 

 more simply in Ostrea., inasmuch as the embryonic entoderm here 

 consists, even at an early stage, of a large number of cells (Fig. 

 14 A-D). In Teredo, on the contrary, the two large entoderm-cells 

 are retained for a very long time, only a few smaller entoderm-cells 

 becoming abstracted from them (Fig. 15 B). The development 

 of the enteric cavity and its close connection with the stomodaeal 

 invagination thus occur later (Fig. 15 U).* Consequently, the in- 

 testine of Teredo can only become capable of functioning at a much 



Fig. 15. — A-C, embryos and larva of Teredo (after 

 Hatschek). The entoderm-cells are lightly and the 

 mesoderm-cells more darkly dotted ; a, anus ; d, 

 rudiment of the enteron ; dm, dorsal, vm, ventral 

 retractor muscles ; m, mouth ; s, shell-gland ; ah, 

 shell. 



* The statement of Brooks that, in the American Oyster, the blastopore 

 closes and the mouth and anus appear as new structures in no way connected 

 with it, cannot be reconciled with the account given by Horst. We should 

 then have a condition such as is found in the fresh-water Lamellibranchia 

 {p. 40). Such a condition would have to be regarded as a specialised one, and 

 we should therefore the less expect to find it in the free-swimming larvae of 

 the marine Lamellibranchia. 



