THE DEVELOPMENT OP' THE FORM OF THE LARVA. 93 



recognised early. These lie rather near the ciliated ring and arise as 

 depressions of the ectoderm which become detached from the latter 

 as closed vesicles. The pedal ganglia also develop as paired thicken- 

 ings of the ectoderm near the otocysts ; at a later stage, they also 

 become detached through delamination. In the middle line of the 

 foot, there seems to be an invagination which perhaps corresponds to 

 the pedal gland described fin - Chiton. 



While these changes are taking place in the post-oral part of the 

 larva, the pre-oral section which, in consequence of the preponderance 

 of the former region, appears comparatively reduced, also under- 

 goes modification. Tims, two ectodermal depressions appear close to 

 the ciliated tuft ; these at first are shallow, but deepen more and 

 more (Fig. 37 .4 and B, eg) and eventually give rise to two closed 

 vesicles which are the paired rudiment of the cerebral ganglion. 

 The cells lining these depressions are, at first, directly continuous 

 with the ectoderm of the cephalic pole, the two depressions being 

 connected together by the cells surrounding the apical ciliated tuft, 

 and thus they represent a common brain-rudiment. The invagina- 

 tions, which have become tubular, grow in further and further until 

 they reach the walls of the stomodaeum (Fig. 37 B). At the same 

 time, by the active proliferation of their cells, they become con- 

 siderably thickened. They also finally become detached from the 

 ectoderm (C) and undergo differentiation into fibrous masses and 

 ganglionic cells, so that there is no room for doubt as to their 

 ganglionic character. Only at a later stage does a commissure form 

 between the two halves of the ganglion which now lose their vesicular 

 character. 



The pedal ganglia, as above shown, arise- by delamination from the ectoderm, 

 while the cerebral ganglia originate as invaginations. This is somewhat re- 

 markable, since the cerebral ganglia arise, as a rule, through delamination, 

 in other Mollusca. Considering the greater contractility of the larva, the 

 presence of such invaginations suggests a more or less temporary infolding oi 

 the surface. Kowalevsky assumes that these ganglia first arose as a surface 

 thickening, and explains the invagination of the ganglionic rudiment as due 

 to the absence of room for surface-expansion owing to the limitation of the 

 pre-oral area by the forward con -'-titration of the ciliated ring. The develop- 

 ment of the cerebral ganglion in Dentalium recalls the condition which we 

 shall find in various Gastropods, where it undoubtedly arises by invagination 

 (p. 191). Since, in these latter cases, we have to do with more specialised 

 forms, it would be desirable, in instituting a comparison with Dentalium, to 

 ascertain in what way the cerebral ganglion arises in the more primitive 

 Gastropoda, especially in the Diotocardia. 



