THE GASTROPODA 



101 



primitively aquatic and 

 The organs of aquatic 



forms, passes into the kidney by means of a portal system, and the 

 efferent renal vein generally joins the rectal sinus or is carried 

 direct to the afferent branchial sinus (Valvata). The venous blood 

 of the kidney is therefore carried to the respiratory organs before 

 it is returned to the heart; but in some Streptoneura (Vermetus, 

 Littorina, Cydostoma) and in certain Pulmonates the blood is carried 

 direct to the auricle without passing through the respiratory 

 apparatus. 



The respiration of Gastropods is 

 remains so in the majority of forms, 

 respiration consist of a pair of leafy 

 expansions of the mantle, situated in 

 the pallial cavity and called ctenidia. 

 Each ctenidiurn is the homologue of a 

 single branchia of Chiton (Fig. 28, B, g\ 

 of Nautilus (Fig. 276), or of Nucula 

 (Fig. 206), but most usually only one, 

 namely, that of the topographically left f. 

 side, persists (Figs. 82 and 85). It is 9 

 only in the more primitive Ehipido- 

 glossa viz. the Pleurotomariidae (Fig. 

 127), the Fissurellidae (Fig. 81), and 

 the Haliotidae that a pair of ctenidia 

 persists. In the Fissurellidae these two 

 organs are quite symmetrical and of 

 equal importance, but in the Pleuro- 

 tomariidae and Haliotidae the topo- 



d. 



81. 



. . ,, . . ... . ,-. Dorsal view of a specimen of 



graphically right CtenidlUm IS Smaller Fissurella from which the shell has 





than the left, and in all other Gastropods 

 there is only a single ctenidium, that of 



the right side having completely dis- (archaic right) gill-plume ; , reflected 



j T ii ,1 cu J.T- mantle-flap ; fi, the fissure or hole in 



appeared. In all the Streptoneura, the the mantle -flap traversed by the 



Pleurobranchidae, Gastropteron, and the ^^Sa^^^S^'i 



Lophocercidae each Ctenidium is formed h > left (archaic riht)renal aperture; 

 - *o . 1 , . , P, snout. (After Lankester.) 



of flattened respiratory filaments which 



lie parallel to one another and are disposed perpendicularly along 

 one or two faces of a branchial axis. Such a ctenidium is called 

 "pectinate." In the Opisthobranchs the only Euthyneura that 

 possess ctenidia the ctenidium is a simple flat and projecting 

 tegumentary lamina, transversely folded from its base to its ex- 

 tremity in such a manner that the ridges of one face correspond to 

 the furrows of the other face : such a branchia is called "plicate." 



Among the dibranchiate Aspidobranchs, Pleurotomaria, the 

 Fissurellidae, and the Haliotidae have two rows of pectinations to 

 each ctenidium, one on either face of the branchial axis (Fig. 81). 

 Each ctendium is therefore formed like that of Chiton, Nautilus, or 



