XII 



PHYLUM MOLLUSCA 



737 



is a specially contractile part of the principal sinus. Two tubes open near 

 the anus, the right one acting as a gonoduct, the left (k) entirely renal in 

 function. The sexes are distinct. There is an elongated unpaired gonad (g. ), 

 divided by lateral incisions into a number of lobes, occupying all the posterior 

 and dorsal parts of the body. Anteriorly it narrows to form the gonoduct. 



The nervous system consists of paired cerebral, 

 pleural, pedal, and visceral ganglia ; the cerebral 

 ganglia are situated close together. There are no 

 eyes or statocysts. 



In the gastrula stage the embryo, which is provided 

 with cilia, becomes free. The ciliated cells are 

 arranged in a characteristic manner in three rows 

 which, at first situated close together about the 

 middle of the body, become shifted at a later stage 

 near the apical pole, and amalgamated into a broad 

 band representing the pre-oral circlet of other 

 molluscan larvae ; at the same time a bunch of cilia 

 previously developed at the apical pole becomes 

 more conspicuous and a considerable part of the 

 general surface covered with more delicate cilia. 

 The blastopore, at first terminal, is shifted forwards 

 on the ventral surface until it comes to be imme- 

 diately behind the ciliated circlet. At its anterior 

 end an invagination gives rise to the mouth and 

 stomodseum. 



The larva (Fig. 661) has now attained the stage of 

 a trochophore, in which, however, both apical plate 

 and primitive nephridia are wanting. A shell-gland 

 is developed, and soon the rudiment of the shell 

 appears. The post-oral region, at first inconsiderable 

 in size, soon undergoes an increase, until it forms 

 eventually by far the longest part of the body, while 

 the pre-oral region almost completely aborts. When 

 the post- oral region has attained a certain size, there 

 are developed on it two lateral folds, the rudiments 

 of the mantle (B), which grow inwards towards the 

 middle ventral line, and later on unite by their free 

 margins. The pre-oral circlet or velum changes its form at first it is 

 conical, later it becomes plate-like, and is then gradually reduced, the 



FIG. 660. Dentalium, 

 anatomy, a. anterior 

 aperture of mantle ; /. 

 foot ; g. gonad ; k. 

 kidney ; /. digestive 

 gland. (From the 

 Cambridge Natural 

 History, after Lacaze- 

 Duthiers.) 



FIG. 661. Veliger of Dentalium. A, longitudinal section of a larva 14 hours old ; B, larva 

 of 37 hours ; C, longitudinal section of larva of 34 hours, m. mouth ; v, v, velum. (From 

 Cookc, after Kowalewsky.) 



