706 



ZOOLOGY 



SECT. 



lobed processes or pinnate palpi. Further back are a pair of tentaculiferous lobes, 

 each bearing a large number of filiform tentacles, which are probably respirator}' 



in function. The mouth leads into a buccal cavity 

 containing an odontophore. Connected with the mesen- 

 teron is a large bilobed liver (/. ). The anus is situated 

 ventrally l)ehind the Imse of the foot. The vascular 

 system is extremely simple, consisting 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 principal sinus. Two nephridia 

 open near the anus, the right one acting as a gonoduct, 

 the left (/>) entirely renal in function. The sexes are 

 distinct. There is an elongated unpaired gonad ((/), 

 divided by lateral incisions 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 iiephridium. 



The nervous system consists of paired cerebral, pleural, 

 pedal, and visceral ganglia ; the cerebral ganglia are 

 situated close together. There are no eyes or otocysts. 



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 nearer the apical pole, 

 and amalgamated into a broad band representing the 

 prse-oral circlet of other molluscan larvit ; at the same 

 tin if a bunch of cilia previously developed at the apical 

 pole becomes more conspicuous, and a considerable part 

 of the general surface becomes covered with more 

 delicate cilia. The blastopore, at first terminal, becomes 

 shifted forwards on the ventral surface until it comes 

 to be immediately behind the ciliated circlet. At its 

 anterior end an invagination gives rise to the mouth and 

 stomodaeum. 



The larva (Fig. 614) is now attaining the stage of a trochosphere, in which, 

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



Fn;. 1,1:1. Dentalium, 



anatomy. . anterior 

 aperture of mantle ; f. 

 fc ic >t ; g. gonad ; k, neph- 

 ridium ; /. liver. (From 

 the Cambridge Sut,<,;<i 

 Hixtin-i/, after Lacaze- 

 Duthiers.) 



Fie;. iii4. Veliger .if Dentalium. A, longitudinal section of a larva 14 hours old: B, larva 

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

 . Cooke. after Kowalewsky.) 



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

 inconsiderable 'n size, soon undergoes an increase, until it forms eventually by 



