74 COMPARATIVE ANATOMY chap. 



rectum projects into the small mantle cavity. This anal cone is not median, but 

 is distinctly shifted to the right. To its right and left lie the nephridial apertures, 

 raised on short conical papillae. There is no separate genital aperture. In some 

 forms {Tcdura, Scurria, Acmtca) one ctenidium is found attached to the mantle, on 

 the left side of the pallial cavity. Further details as to the gills in the PatelUdcc 

 will be given later on. We further find, on the floor of the cavity, on each side, 

 ti-aces of an osphradium in the shape of a small patch of sensory epithelium, which 

 may be raised on a prominence. It is doubtful if the prominence found in Tatclla 

 close to each osphradium, containing a blood sinus divided up by septa, can be 

 considered as a rudimentary gill. These prominences rise from the flmr of the 

 mantle cavity, whereas in Tectum, for example, in which a true gill still occurs on 

 the left, it lies far removed from the left osphradium, in the usual position on the 

 roo/' of the cavity, i.e. on the inner surface of the mantle. 



h. Monotocardia. — In this division, the numerous forms of which show little 

 variety of organisation, the arrangement of the pallial complex is very uniform. 

 The single genital aperture is always distinct from the single nephridial ai)erture. 

 The position of the organs in the spacious pallial cavity (Fig. 71), from right 

 to left, is as follows : — 



1. To the extreme right, lies the aflerent duct of the genital organs (ovary or 

 seminal duct), which runs more or less far forward, in the mantle cavity. 



2. In contact with this, but quite on the roof of the cavity, is the rectum. 



8. To the left of the rectum, far back in the base of the mantle cavity, lies the 

 slit-like nephridial aperture, which pierces the wall separating the cavity from the 

 renal organ behind and above it. Exceptions occur in Paludina and Valvata, in 

 which this aperture is shifted forward to the end of a urinary duct which runs on 

 the mantle. 



4. On the roof of the mantle cavity are found the hypobranchial glands (mucous 

 and jjurple glands), which are developed in varying degrees. 



5. Quite to the left, and also on the roof of the cavity, the ctenidium, feathered 

 on one side (the left {vr) oi Haliotis and Fissurella), at whose base, deep back in the 

 cavity, the pericardium is visible with the ventricle and auricle seen through it. 



6. Finally, to the extreme left, lies the osphradium, which is always well 

 developed and sharply circumscribed, and is either filamentous or feathered on two 

 sides, and attached to the roof of the pallial cavity. 



The position of the organs in the pallial complex of the Hctcropoda, certain forms 

 of which, such as Atlanta, are closely related to the other Monotocardia, requires to 

 be re-investigated. Tlie osphradium lies at the base of the gill. 



2. Fulmonata. 



In the Pulmonata, the single or double ($ and S ) genital aperture (Fig. 72) no 

 longer belongs to the pallial complex, but lies outside the mantle cavity laterally on 

 the head or neck. In Oncidiimi the male aperture lies anteriorly under the right 

 tentacle, the female posteriorly, near the anus. 



Bearing in mind that the mantle or pulmonary cavity communicates Mith the 

 exterior only by means of the respiratory aperture lying on the right, we have the 

 following arrangement of the pallial complex as typical (excluding such aberrant 

 forms as Daudcbardia, Trstacrlla, and Oncidimn). 



1. On the extreme right of the pulmonary cavity lies the rectmn, the anus 

 opening in the resjiiratory aperture. 



2. On the roof at the back of tlie cavity lies the nephridium (kidney). 



3. To the left, near the kidney, also far up in the cavity, and on its roof, lies 



