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 respiratory 

 in function. The mouth leads into a buccal cavity 

 containing an odontophore. Connected with the mesen- 

 teroii is a large bilobed liver (I. ). The anus is situated 

 ventrally behind the base of the foot. The vascular 

 system is extremely simple, consisting of sinuses without 

 definite walls, and there is 110 distinct heart, though in 

 the neighbourhood of the rectum there is a specially 

 contractile part of the principal sinus. Two iiephridia 

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

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

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

 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 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 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 

 pree-oral circlet of other molluscaii larva? ; at the same 

 time a bunch of cilia previously developed at the apical 

 FIG. 613. Dentalium, pole becomes more conspicuous, and a considerable part 

 a^rture 'of ma^tle^T f the eneral surface becomes covered with more 

 foot ; g. gonad ; k, neph- delicate cilia. The blastopore, at first terminal, becomes 

 ridium ; /. liver. (From shifted forwards on the ventral surface until it comes 

 S^SrSr ilSf. to be immediately behind the ciliated circlet. At its 

 Duthiers.) anterior end an imagination gives rise to the mouth an 



stomoda?um. 



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

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



FIG. 614. Veliger of Dentalium. A, longitudinal section of a larva 14 hours old ; B, larv; 

 . of 37 hours ; C, longitudinal section of larva of 34 hours ; -/,*. mouth ; v. r. velum. (Fror 

 Cooke, after Kowalewsky.) 



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

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



