RESPIRATOR V ORGANS — CTENIDIA. 
801 
Tlie fiuictioii of respiration is to sujiply oxygen to the blood and 
tissues and to eliminate the gaseous products of decay, and essentially 
consists in the exposure of the impure venous blood to the vivifying 
influence of air, or water containing air, so that oxygen is absorbed 
and carbonic acid eliminated. 
This result in the mollusca is accomplished chiefly by brauchitie in 
the ai^uatic species and by a lung cavity in the terrestrial forms. 
Auxiliaiy to the respiration carried on in these specialized organs, 
there is cutaneous respiration and tissue or internal respiration. 
Branchle (^pdi'yia, the gills of a fish) or Gills, the specialized 
organs for the respiration of water, are differentiations of the integu- 
ment, and are in strict correlation with an a(jnatic life, and therefore 
of gi’eater extent and complexity than the lung of the Pulmonates, 
to compensate for respiring a medium containing so little oxygen. 
The Ctenidia (Krei-tStoi', a little comb) or primitive molluscan gills 
are typically symmetrical and paired free plume-like structures, 
although it is only in some of the Pelecypods and Zygobranchiate 
Gastropods that their original character and arraugemeut are preserved. 
They are assumed to have arisen 
as simple ridges at each side of 
the body, along a line extending 
from the mouth to the anus, known 
as the Lophophoral (Xd(/)os, a 
plume ; c^dpcoj, to carry) line on 
account of its relation to the oral 
disk or lophojihore of the Polyzoa, 
etc. These ridges, by elongation, 
each give rise to a row of hollow, 
^ciliated respiratory processes, 
usually stiffened by chitinous rods, 
along one edge, each filament con- 
taining afferent and efferent axial vessels, and bearing at each side 
numerous delicate and perpendicular vascular proces.ses, lined in- 
teriorly with connective tissue and supported ly muscular trabeculai, 
which under nervous influence, relax and contract, as.sisting to 
alternately receive and expel the blood. In the ancestors of most 
of the active Gastropoda the lateral gill-processes and the area 
occupied by them, from various causes, become more restricted and 
tend also to be relegated towards the rear of the animal. 
Fig. 590. — Anodonta cygnea (L.) with 
mantle, gills and labial palps removed to 
show the Lophophoral line (after Lankester). 
a. a. line of attachment of the anterior 
palp and /./. of the posterior palp, con- 
tinuous with l.c. the line of attachment of 
the left ctenidium or gill, jointly forming the 
Lophophoral line ; a. ad. anterior adductor; 
p.ad. posterior adductor; iii. mouth foot; 
n.o. nephridial opening ; g.o, reproductive 
orifice. 
