72 



COMPARATIVE ANATOMY 



CHAP. 



In order to prevent confusion, the hypothetical original position of each organ 

 will be denoted by ur ( = originally right) and ul ( = originally left) in brackets. 



In the upper part of the mantle cavity in Fissu-rella, beneath the median 

 aperture in the mantle and shell, lies the anus, and immediately to its right, the 

 right (ul) nephridial aperture, immediately to its left the left (ur) nephridial 

 aperture ; the right (ul) and left (itr). ctenidia, again, lie symmetrically to the right 

 and left. There are no distinct osphradia, and the genital apertures are wanting as 

 the genital gland opens into the right nephridium. 



Haliotis. The mantle cavity has here shifted to the left, and the rectum, 

 attached to the mantle fold, runs forward some way through it, so that the anus is 

 at a considerable distance from the posterior apex of the cavity. On the right of the 

 rectum lies the right (ul), and to its left the left (ur) ctenidium, both fastened to 

 the mantle, and stretching far forward. The right and left nephridial apertures 

 lie near the bases of the ctenidia, in the upper and posterior part of the mantle 



FIG. 70. The same specimen from the left 

 side. Lettering as before ; o, mouth. 



FIG. 69. Anterior portion of Patella, from 

 above, after removal of the mantle fold (after 

 Ray Lankester). a, Tentacle ; b, foot ; c, 

 pedal muscles (shell muscles) ; d, osphradia ; 

 e, mantle fold ; /, aperture of the right nephri- 

 dium ; g, anal papilla and anus ; h, papilla and 

 aperture of the left nephridium ; i, left nephri- 

 dium ; A-, right nephridium ; I, pericardium ; 

 n, digestive gland (liver) ; m, cut edge of the 

 mantle ; p, snout. 



cavity. Between the rectum and the left ctenidium, also on the mantle, is found 

 the long, well-developed hypobranchial gland (mucous gland), which stretches as far 

 forward as the gill. Only a small portion of the gland lies to the right between 

 the rectum, as far as it runs, and the right ctenidium. There are two osphradia 

 which run as bands along the axes of the ctenidia facing the mantle cavity. 



Turbinidae and Trochidse. Only the left (ur) ctenidium of Haliotis is here 

 retained ; it lies far to the left on the roof of the mantle cavity, i.e. on the mantle. 

 The rectum runs far forward along this roof. Two nephridial apertures lie on 

 papillae in the base of the cavity, at the sides of the rectum. The hypobranchial 

 gland is found in various stages of development, the highest being attained in the 

 Turbinidce. It is largest between the rectum and ctenidium, i.e. between the 

 right side of the latter and the left side of the former. In the Turbinidce, however, 

 a portion of it lies to the right of the rectum. There is a diffuse osphradium on the 

 axis of the gill. 



Neritina. There is here only one gill (the left (ur) in Haliotis) shifted somewhat far 

 to the right. The rectum lies asymmetrically to the right in the respiratory cavity, 



