CONCHIFERA. - 397 
and to the siphonal partition. Each gill is composed of two 
laminee, divided internally into a series of parallel tubes, indicated 
outside by transverse lines; these tubes open into longitudinal 
channels at the base of the gills, which unite behind the posterior 
adductor muscle at the commencement of the exhalent siphon (c). 
Examined by the microscope, the gill laminz appear to be a 
network of blood-vessels whose pores opening into the gill-tubes, 
are fringed with vibratile cilia. These microscopic organs perform 
most important offices; they create the currents of water, arrest 
the floating particles, and mould them, mixed with the viscid 
secretion of the surface, into threads, in the furrows of the gill, 
and propel them along the grooved edge of its free margin, in 
the direction of the mouth; they are then received between the 
palpi in the form of rayvelled threads. (Alder and Hancock.) 
In Mya, therefore (and in other burrowers), the cavity of the 
shell forms a closed branchial chamber, and the water which 
enters it by the respiratory siphon can only escape by passing 
through the gills into the dorsal channels, and so into the 
exhalent siphon. In the river-mussel the gills are not united 
to the body, but a slit is left by which water might pass into 
the dorsal channel, were it not for the close apposition of the 
parts under ordinary circumstances (Fig. 208 b). The gills of 
the oyster are united throughout, by their bases, to each other 
and to the mantle, completely separating the branchial cavity 
from the cloaca. In Pecten the gills and mantle are free, but 
the ‘‘dorsalchannels” still exist,and carry out the filtered water. 
In some genera the gills subserve a third purpose; the 
oviducts open into the dorsal channels and the eggs are received 
into the gill-tubes and retained there until they are hatched. 
In the river-mussel the outer gills only receive the eggs, 
with which they are completely distended in the winter months 
(Fig. 208, 0, 0). In Cyclas the inner gills form the marsupiwn, 
and only from 10 to 20 of the fry are found in them at one 
time ; these remain until they are nearly a quarter the length of 
the parent.* 
The valves of the Conchifera are bound together by an elastic 
ligament, and articulated by a hinge furnished with interlock- 
ing teeth. The shell is closed by powerful adductor muscles, 
but opens spontaneously by the action of the ligament, when the 
animal relaxes, and after it is dead. 
Each valve isa hollow cone, with the apex turned more or 
* Some other particulars respecting the organisation and development of bivalve 
shell-fish are given in ChapterI. For an account of their vascular system see Milne- 
Edwards, An, Sc. Nat. 1847, tom, viii. p. 77. 
