244 



MANUAL OF THE MOLLUSCA. 



of parallel tubes, indicated outside by 

 transverse lines; tbese tubes ojien 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 ap- 

 pear to be a network of blood-vessels 

 whose pores opening into the gill-tubes, 

 are fringed with vibratile cilia. These 

 microscopic organs perform most impor- 

 tant offices ; they create the currents of 

 water, arrest the floating particles, and 

 mould them, mixed with the viscid secre- 

 tion 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 Ha/i- 

 cock.) 



In Mtja, therefore (and in other bur- 

 rowers), the cavity of the shell forms a 

 closed branchial chamber, and the water 

 which enters it by the respiratory sijihon 

 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. 171, '5'). 

 The gLUs 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. 



Fig. 170. Mya arenaria. 



* Mya arenaria, L. (original, from specimens obtained at Southend, and commu- 

 nicated by Miss Hume). The left valve and mantle lobe and half the siphons are re- 

 moved, a, a', adductor muscles ; 6, body; c, cloaca; /, foot; g, branchiae; h. heart; 

 m, cut edge of the mantle; o, mouth; s, s\ siphons; t, labial tentacles; v, vent. 

 The arrows indicate the direction of the currents; the four rows of dots at the base 

 of the gills are the orifices of the branchial tubes, opening into the dorsal channels. 



