200 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



the mantle. This space is all that remains of the pallial cavity. 

 It contains the single ctenidium and the osphradinm or water- 

 testing organ, and into it discharge the reprodnctive apparatus, 

 the kidney, the anus, and certain glands to be described later. 



The Ctenidium, The ctenidium (Fig. 7, ct.), as in most 

 Tectibranchs, is a lobate plume-like structure which is attached 

 by a broad base to the postero-lateral portion of the floor of 

 the pallial cavity. Its free portion projects backwards and 

 slightly outwards into the cavity, but it can be rotated on its 

 axis or even thrust upwards above the edge of the mantle 

 shelf, the last-named position occurring when the water 

 becomes deficient in oxygen. Along the antero-dorsal convex 

 side of the ctenidium runs a mid-rib, formed by the efferent 

 branchial vessel. On each side of it are ranged the pinnules 

 of the gill. The afferent branchial vessel forms a similar mid-rib 

 on the postero-ventral concave side of the ctenidium. (Vide 

 Circulation.) 



The Osphradium. (Spengel's Olfactory Organ.) Immedi- 

 ately anterior to the attachment of the ctenidium and lyino- 

 in the body wall is situated an organ whose function is to test 

 the water approaching the ctenidium. This, the osphradium, 

 is visible as a yellow patch in the living animal, but is often 

 difficult to find in a preserved specimen. (Vide Sense Organs.) 



Genital Aperture. The common genital aperture (Fig. 7, 

 c. gen. ap.) is a small crescentic opening situated at the right 

 anterior end of the pallial cavity on its floor, at the level of 

 the anterior limit of the projecting mantle shelf. Its position 

 can be readily determined as it lies at the junction of the 

 pigmented and unpigmented skin of the neck and pallial cavity 

 respectively. Moreover, from it arises the obliquely placed 

 seminal groove which extends forwards to the penis at the 

 base of the right anterior tentacle. 



Nephridial Aperture. The nephridial aperture (Fig. 21, 

 ren. p.) is situated on the under side of the overhanging roof 

 of the pallial cavity immediately posterior to the point of 



