32 LAMELLIBRANCHIA. 



later stage than that of Ostrea. The proctodaeum also seems tx> 

 develop earlier in the latter. In Teredo, according to HATSCHEK, 

 the terminal portion of the intestine arises as an ectodermal invagina- 

 tion at the posterior end of the body (proctodaeum), which afterwards 

 becomes connected with the enteron (Fig. 15 (7, a). 



Even before the processes just described are completed, other im- 

 portant changes, especially affecting the external shape of the embryo, 

 have taken place. At the time of the formation of the stomodaeum, 

 the ectoderm began to separate from the entoderm, thus giving rise 

 to the primary body-cavity and at the same time causing a striking 

 alteration in the shape of the embryo (Figs. 14 and 15). The latter, 

 which hitherto was almost egg-shaped, now broadens anteriorly, the 

 pre-oral part assuming the shape of a somewhat flattened cupola, 

 while post-orally the body tapers slightly ; in fact, the larva assumes 

 the shape with which we became familiar in the Annelidan Trochophore 

 (Vol. L, p. 265, etc.). 



During this alteration in the shape of the embryo, the ciliation 

 characteristic of the Trochophore also appears, two rows of cells lying 

 in front of the mouth and encircling the cephalic area becoming 

 covered with cilia (Fig. 15 A). The pre-oral ciliated ring consisting 

 of a double row of cells thus arises, but, in the following ontogenetic 

 stages of Teredo, this is the less distinct, as the whole body becomes 

 covered with cilia, most of which are lost again later. Then, only 

 the biserial pre-oral ciliated ring persists, while behind the mouth 

 are seen the first indications of a post-oral ciliated ring. These are 

 gradually continued towards the dorsal side until the closed post-oral 

 uniserial ciliated ring is produced (Fig. 18 w, t ). Between the two 

 rings, a zone of more delicate cilia is retained ; this is called by 

 HATSCHEK the ad-oral ciliated zone. Behind the anus, also, a small 

 ciliated area is found. A tuft of strong cilia or a single thick cilium, 

 found in many Lamellibranch larvae in the centre of the cephalic area, 

 makes the likeness to the Annelidan Trochophore, already produced 

 by the form of body and distribution of the cilia, still more striking 

 (Fig. 18). 



While the post-oral ciliated ring and the ad-oral zone no doubt 

 assist in the capture of food, the pre-oral ring is specially adapted for 

 locomotion. In accordance with this function it is always found 

 specially well developed in the free-swimming larvae, in which the 

 post-oral ring and the other ciliation may degenerate. This important 

 locomotory organ attains in many larvae so great a development that 

 the anterior part of the body carrying it projects beyond the rest of 



