THE GASTROPODA IOI 
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. 
The respiration of Gastropods is primitively aquatic and 
remains so in the majority of forms. The organs of aquatic 
respiration consist of a pair of leafy 
expansions of the mantle, situated in 
the pallial cavity and called ctenidia. 
2, 
Each ctenidium is the homologue of a 4 hy Ai 
single branchia of Chiton (Fig. 28, B, q), q \ () Ff 
of Nautilus (Fig. 276), or of Nucula  . HV I\Z i 
. d WANG li 
(Fig. 206), but most usually only one, ‘ J Hl. 
namely, that of the topographically left | (G \ 
side, persists (Figs. 82 and 85). It is aa aN {] fy 
only in the more primitive Rhipido- 7%; ees 
glossa—viz. the Pleurotomaridae (Fig. > wa 
127), the Fissurellidae (Fig. 81), and i Ce 
the Haliotidae—that a pair of ctenidia on ay 
persists. In the Fissurellidae these two : x fs 
organs are quite symmetrical and of 5 SSS ff 
equal importance, but in the Pleuro- a ie 
tomariidae and Haliotidae the topo- 
graphically right ctenidium is smaller 
than the left, and in all other Gastropods 
there is only a single ctenidium, that of 
the right side having completely dis- 
appeared. In all the Streptoneura, the 
Pleurobranchidae, Gastropteron, and the 
Dorsal view of a specimen of 
Fissurella from which the shell has 
been removed, and the anterior area 
of the mantle-skirt has been longi- 
tudinally slit and its sides reflected. 
a, cephalic tentacle; b, foot; d, left 
(archaic right) gill-plume; e, reflected 
mantle-flap ; ji, the fissure or hole in 
the mantle-flap traversed by the 
longitudinal incision; f, right (ar- 
chaic left) renal aperture; g, anus ; 
h, left (archaic right) renal aperture ; 
Lophocercidae each ctenidium is formed PR aiGAE EEL 
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, Pleuwrotomaria, 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 
