RESPIRATION IN PELEOYPODA AND GASTROPODA. 
species, however, are Eulainelliliranchs, in wliich the filanieiitary 
processes have become fused together and form more or less complexly 
perforated and folded gill plates, lying in a branchial chamber ; the 
interlamellar cavities, in some species, serving as marsupial or in- 
cubatory pouches for the protection of the eggs or young. 
The delicate branchial tissue is easily detached in minute Hakes by 
compression or friction, the detached fragments moving about like 
living Infusoria liy the action of the cilia clothing their surface. 
Miiller actually described these motile particles as species of his 
Infusorian genera, Trichodd and lAUicophra. 
In Gastropods the gills, though simpler in character, are constructed 
on the same iilan as those of the Pelecypods and occupy a similar, 
though more restricted, position, hut, owing to body torsion, the 
primitively left gill has become 
atrophied in all our species, al- 
though still present in a more 
or less primitive form in the 
Fig. 592.— Ctenidium or gill of a Monoto- r/ ^ i • , rm 
cardiate Gastropod showing its pinnate character /iyC'ObrailCllUlUl. lllG 2'llls cU'C 
(niter mng). ° 
cl. ctenidium or gill with efferent Mood ves.sel USUally HOW Ot a Seinipilinate 
le.nding to the auricle, rr.; f'. ventricle terminating t , -.I j. 1 , • c i 
in rt < 1 . anterior aorta and /.«. posterior aorta. CliaraCter Wltll the aXlS tUSCd tO 
the roof of the mantle chamber, the respiratory processes being 
arranged parallel to each other, lilce the teeth of a comb, and freely 
projecting into the respired -water. 
In the branchiate species resjiiration is [irobalily more or less 
continuous when in process, more especially in the Pelecypoda, in 
which the alimentary function is so intimately combined with that 
Fig. 593. — Vnio pictoy-nni (L.) with right valve and mantle removed to show the courses of the 
water currents during respiration (after 6lt). 
The arrows indicate the courses traversed by the inspired water. 
of respiration. The currents produced by the dense ciliary invest- 
ment enter by the inhalent aperture, and are aftirmed to How in a 
steady and continuous stream along more or less definite and precise 
