THE GASTROPODA 



101 



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, Cyclostoma) and in certain Pulmonates the blood is carried 

 direct to the auricle without passing through the respiratory 

 apparatus. 



of Gastropods is 



majority of forms. 



of a pair of leafy 



mantle, situated in 



and called ctenidia. 



The 



respiration 

 remains so in the 

 respiration consist 

 expansions of the 

 the pallial cavity 



primitively aquatic and 

 The organs of aquatic 



A 



Each ctenidium 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 

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

 only in the more primitive Rhipido- 

 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- 



. Jjj Dorsal view of a specimen of 



graphically right Ctenidium IS Smaller Fissurella from which the shell has 



TV ,i i r, -i it ,i r> j been removed, and the anterior area 



than the left, and in all Other Gastropods O f the mantle-skirt has been longi- 



tViPrP 10 nnlv i eino-lp pfpnirlinm thaf nf tudinally slit and its sides reflected. 



mere is only a single cteniamm, tnat 01 ^ cepha ^ ic tentacle; &, foot; d, left 



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



, T ni n i mantle-flap ; fi, the fissure or hole in 



appeared. Ill all the btreptoneura, the the mantle -flap traversed by the 



Pleurobranchidae, Gastropterm, and the K^rJ^^j'TLS; 



Lophocercidae each Ctenidium is formed Meft (archaic right) renal aperture ; 

 - *T , . . i_-i?' snout - < After Lankester.) 



ot 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 



