XII 



PHYLUM MOLLUSCA 



757 



situated on a short oral proboscis, and is sometimes surrounded by lobed pro- 

 cesses or pinnate palpi. Further back are a pair of tentaculiferous lobes, each 

 bearing a large number of filiform tentacles, which are 

 probably respiratory in function. The mouth leads into 

 a buccal cavity containing an odontophore. Connected 

 with the mesenteron is a large bilobed digestive gland (I. ). 

 The anus is situated ventrally behind the base of the 

 foot. The vascular system is extremely simple, con- 

 sisting of sinuses without definite walls, and there is 

 no distinct heart, though in the neighbourhood of the 

 rectum there is a specially contractile part of the prin- 

 cipal sinus. Two nephridia 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 (#.), divided by lateral in- 

 cisions into a number of lobes, occupying all the posterior 

 and dorsal parts of the body. Anteriorly it narrows 

 to form a duct opening into the right nephridium. 



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 immediately 

 behind the ciliated circlet. At its anterior end an in- 

 vagination gives rise to the mouth and stomodseum. 



The larva (Fig. 651) has now attained the stage of a trochophore, in which, 

 however, both apical plate and primitive nephridia are wanting. A shell-gland 



Fio. 650. Dentalium, 



anatomy, a. anterior 

 aperture of mantle ; /. 

 foot; (i. gonad ; k, neph- 

 ridium ; I. digestive 

 gland. (From the Cam- 

 bridge Natural History, 

 after Lacaze-Duthiers.) 



FIG. 651. 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. i. mouth ; r, v. velum. (From 

 Cooke, after Kowalewsky.) 



is developed, and soon the rudiment of the shell. The post-oral region, at first 

 inconsiderable in size, scon undergoes an increase, until it forms eventually by 



