424 " THETIS " SCIENTIFIC RESULTS. 



right side of the cavity, parallel with and mesial to the rectum. 

 Its colour is a dirty bi'ick-red, approaching to brown, and on 

 the tops of the ridges grey. The excreting area is much increased 

 by the throwing of the surface into I'idges ; of these ridges the 

 most prominent are those which run transversely and diagonally to 

 the long axis of the organ, and between them are smaller ridges 

 arranged longitudinally. 



Next to the mucous gland, and parallel thereto, is the rectum 

 (i'ect.) ; this partly overlies the uterus (ut.), which is the organ 

 nearest to the body on the right side. These organs will be 

 described with the systems to which they belong. 



2. Digestive System. 

 (PI. xlii., figs. 1 and 2.) 



The absence of a snoui has already been noted. The orifice of 

 the proboscis-sheath lying below the tentacle lobe is beset with 

 some eight or nine short thick papillae. 



The prohoscis-sheath (ph. sh.) extends back for some little 

 distance and opens into the proboscis-sac. This sheath is not 

 evaginable, but is fused dorsally with the body wall ; ventrally 

 it is attached by numerous muscle strands to a spongy connective 

 tissue which completely fills the body cavity below it ; its walls 

 are thick and tough. The cuticle of the inner wall at the 

 posterior, or deeper end of the sheath, is raised up into circularly 

 disposed filaraentose lamellae. As to their function, one can only 

 suggest that they serve to clean the px'oboscis as it is retracted, 

 and so prevent dirt from getting into the proboscis-sac. 



The prohoscis-sac (pb. sac) is an ovoid compartment with 

 thin but tough walls, in which the proboscis is coiled. To 

 the outside of the "sac," just close to the aperture from 

 " sheath " and on the right hand side, there is attached a band 

 of muscle (p. mus.) which passes back beside the sac, becomes 

 suddenly thickened when the posterior end thereof is reached, 

 and is attached to the body wall on the right side near the pos- 

 terior end of the body cavity ; but for this muscle the sac lies 

 free in the ca%'ity. 



The proboscis (pb.) is A'ery long, and lies coiled up within the sac ; 

 its walls are thick and muscular ; it is attached to the left side 

 of the sac near the posterior end. 



In typical Prosobranchiata Proboscidea, when the proboscis is 

 fully evaginated there is a portion lying within the proboscis- 

 sheath, and this is the portion which is evaginable. On the other 

 hand, when the proboscis is completely invaginated the anterior 

 end still lies within the sheath.* 



* Vide " Acrecbolic introvert," fig. 48 in Lankester's article on Molhisca 

 — Ency. Brit. , ed. 9, xvi. , MoUusca, 1SS4- 



