92 SOLENOCONCHA. 



of a certain character typical of the Molluscan larva. Thus, a dorsal 

 invagination of the ectoderm (Fig. 36, *<7) becomes differentiated at a 

 very early stage (^4), then deepens and flattens out again later ; this 

 organ, from its development and subsequent modification, as well as 

 in its position, is seen to be the shell-gland, a structure peculiar to 

 the Mollusca. A comparison of the figures of the Dentalium larva 

 with those of the Lamellibranch and Gastropodan larvae (Figs. 14, 

 p. 28, 15, p. 31, and Fig. 50, p. 124) will enable the reader without 

 further assistance to recognise the great resemblance in the position 

 of the organs in these different larval forms. As the shell of 

 Dentalium is secreted on the dorsal surface of the posterior section of 

 the body, just where the shell-gland appears, it shows the same 

 manner of origin and shape as the young shells of other Molluscs. 

 It shows special resemblance with that of the Lamellibranchs, since 

 it extends like a saddle from the back on to the two sides of the body, 

 but, whereas the young Lamellibranch shell soon becomes bivalve, 

 the shell of Dentalium remains single, i.e., it remains to a certain 

 extent at ;t stage which, in the Lamellibranchs, was found to precede 

 the bivalve shell (p. 60). 



Before the shell develops, further important changes take place in 

 the free-swimming larva of Dentalium, the post-oral region being the 

 first affected by them. During the early stages, this section is very 

 inconspicuous (Fig. 34 C), but it soon increases in size. This region 

 by its growth gives rise to the greater part of the adult body, the pre- 

 oral section degenerating almost completely. We find in this respect 

 a similarity between Dentalium and the Amphineura (pp. 5 and 6), 

 and when treating of these processes in the latter, they were compared 

 with the corresponding processes of metamorphosis in the Annelida. 



At an early stage, the pre-oral portion of the body becomes 

 somewhat swollen and distinctly marked off' from the post-oral part 

 (Figs. 35 and 36 B). The definitive mouth is derived frqm an 

 invagination lying immediately behind the ciliated ring (Fig. 36, //>). 

 The depression on the dorsal side which is to be regarded as the shell- 

 gland (srl) has already been mentioned. When the post-oral section 

 has increased still further in size, two folds laterally placed arise on 

 it ; these grow out towards the ventral middle line and at a some- 

 what later stage meet, at first near the posterior end (Fig. 35 C, >//). 

 These folds, the free edges of which fuse later, represent the rudi- 

 ment of the mantle which thus rises here very much in the same way 

 as in the Lamellibranchia. The folds enclose a ventral swelling, 

 the foot (Fig. 38 B,f), at the base of which the otocysts are to be 



