742 



ZOOLOGY 



lateral mantle-flap. Usually there is on the inner surface of the 

 mantle a glandular area — the pallial mucus-gland. 



Respiratory Organs. — There are typically two ctenidia, one on 

 the right side and the other on the left, contained in the mantle- 



pr*ob 



Fig. 630. — Pterotracliea scutata. ali. alimentary canal; cten. gills; eye, eye; fi. float; 

 mo. mouth ; pvob. proboscis ; repr. gonad ; sh. shield covering a portion of the dorsalsurface ; 

 «!. sucker. 



cavity ; but in the great majority of the Streptoneura and branchiate 

 Euthyneura the primitively right (actually left) ctenidium alone is 

 retained. In those Gastropoda that possess two ctenidia, and in 

 many forms with only one, the axis of the ctenidium bears two 

 rows of compressed filaments, and is attached only towards its 

 base. But in the majority of those with one ctenidium there 

 is, as in Triton, only a single row of filaments retained, and the 

 organ is attached throughout its length. 



fe. In the Nudibranchs true ctenidia are absent, but their place 

 as breathing organs is taken by a number of secondary branchial, 



oc.lbenc 



lent 



ci-rv 



pulm. 



Fig. 631. — Helix nemoralis. an. anus ; gen. ap. genital aperture ; oc. tent, posterior eye-bearing 

 tentacles ; pulm. opening of pulmonary sac ; tent, anterior tentacles. (After Pelseneer.) 



sometimes simple, sometimes branched or pinnate processes, which 

 are distributed over the dorsal surface, as in Eolis\ or, as in 

 Doris (Fig. 623), form a circlet surrounding the anus; or, as in 

 Pleurophyllidia (Fig. 632), a row on each side beneath the 

 mantle-flap. 



