80 



GOMPARA TIVE A NA TOMY 



The ctenidium, liowever, is wanting. In the base of the pallial cavity, to the left, 

 lies the pericardium, and immediately in front of it the kidney, with a narrow 

 aperture into the cavity ; then follow the osphradium (where this has been found), 

 and, at the extreme right of the cavity, the anus with the anal gland. The mantle 

 gland (hyi)obranchial gland, shield) is found on the roof of the pallial cavity. The 

 genital aperture lies to the right anteriorly in the cephalic region ; from it a 

 ciliated channel or furrow runs dorsally to the aperture of the penis, which lies 

 anteriorly between the fins. 



As compared with the Limacinidcc, i.e. the Thccosomata with coiled shell, the 



Fig. 77.— a, B, C, Three diagrams to illustrate the relation of the Limacinidae to the 

 €avoliniidas (after Boas). A, Limacinidae ; B, hypothetical intermediate stage between 

 the Limacinidae and the CavoliniidaB. The visceral dome twisted 90°. C, Cavoliniidas. All the 



diayraiiis fruin the ventral or posterior side. In A the visceral dome is drawn straij^ht, whereas it 

 is in reality coiled. 1, Right tin (parapodinm) ; •!, foot bent forward ; 3, genital ajierture ; 4, ten- 

 tacular appendage of the mantle edge ; .5, anus ; li, masticatory stomach ; 7, gonad. 



Cavuliii.iidcc and Ci/iiibidivhc, or Tliccosoinata with straight shell, show a very 

 different arrangement of the pallial comiilex, which can only be explained by the 

 supposition that the larger 2>osterior portion of the body (the visceral dome) of 

 the Liinacinidce, with all the pallial organs belonging to it, has twisted round the 

 longitudinal axis of the body 180°, in relation to the cephalic region with the 

 genital apertures belonging to it. Such a twist gives the organs the position they 

 actually occupy in the CavoUiiiidcc and CymbuUidce ; the posterior (ventral) pallial 

 cavity containing, on the left the anus, on the right the pericardium and kidney and 

 the osphradium, the genital aperture occupying its original position to the right. 

 The cause and significance of this twist are at i)resent unknown. 



B. Scaphopoda. 



There is no gill in the posteriorly placed mantle cavity. The anus lies in the 

 middle line above the foot, having a nephridial aperture on each side of it. There 

 are no distinct genital apertures. 



