coxcniFEiiA. 397 



and to tlie siphonal partition. Eacli gill is composed of tvxo 

 larainfx^, divided internallyinto a series of parallel tubes, indicated 

 outside by transverse lines ; these tiibes 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 laminae appear to be a 

 network of blood-vessels whose pores opening into the gill -tubes, 

 are fringed with vibratile cilia. These microscoiDic 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 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 

 jmrts under ordinary circumstances (Fig. 208 &). 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 Fecten 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 

 (Fig. 208, 0, o). In Ci/clas the inner gills form the marsupium, 

 and only fi'om 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 Cci??c7«?/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 orgaiiisation and development of bivalve 

 shell -fish are pi%'en in Chapter I. For an account of their vascular system see Milne- 

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



