COXCHIFEEA. 397 



and to tlie siplional partition. Eacli gill is composed of two 

 laminee, divided internally into a series of parallel tnbes, indicated 

 outside by transverse lines ; tliese tnbes open into longitudinal 

 channels at tbe base of tbe gills, wbicb unite behind the posterior 

 adductor muscle at the commencement of the exhalent siphon (c). 

 Examined by the microscope, the gill laminee 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 ofiELces ; 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 ravelled threads. (Alder and Hancock.) 



In My a, 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 V), 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 '-dorsal channels" 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 

 (Eig. 208, 0, o). In Cyclas the inner gills form the marsupium, 

 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 Co7zcAt/era 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 is a hollow cone, with the apex tui'ned more or 



* Some other particulars respecting the orgatiisation and development of bivalve 

 shell-fish are given in Chapter I. For an account of their vascular system see Milne- 

 Edwards, An. Sc. Nat. 1847, tom. viii. p. 77. 



