Chap. XVI.] THE FRESH-WATER MUSSEL. 313 



examined under a high power, there will be seen : (1) frag- 

 ments of the chitinous rods, (2) ciliated cells, (3) irregular or 

 branched nucleated cells of the lacunar tissue, which supports 

 the ciliated cells and forms the ground substance of the gill, 

 and (4) colourless amoebiform blood corpuscles. 



If the gill from which the square patches have been cut be 

 examined with a hand lens, the inter-lamellar junctions will be 

 seen. They are best seen, however, in the outer gill of a 

 mussel in which this gill has been converted into a brood-pouch 

 for embryos. 



The ceaseless lashing of the cilia of the gills produces the 

 current of water which sets in through the inhalent siphon and 

 escapes through the exhalent siphon. This current passes from 

 the infra-branchial into the supra-branchial chamber, partly 

 through the fenestrse of the gill-lamellse, partly through the 

 large slit-like aperture between the foot and the ascending 

 lamella of the inner gill. It subserves two functions : (1) 

 alimentation by sweeping infusoria, diatoms, and other minute 

 organisms within the reach of the cilia of the mouth cavity; 

 (2) respiration as the water is driven past the gill-filaments 

 and through the fenestrse of the lamellar membrane. 



The Heart and Circulation. The heart (Figs. 91 and 92) 

 consists of a median chamber, the ventricle (ve.\ and two lateral 

 chambers, the auricles (au.). These parts lie within the peri- 

 cardial chamber (p. c.). In a recently killed mussel the slow 

 wave-like contractions of the heart (five or six a minute) 

 may be observed. Seen from above, by opening up the peri- 

 cardial chamber from the dorsal aspect, the ventricle is seen as 

 a thick-walled oval sac, with a tendency to two lateral lobes 

 posteriorly. Through its midst passes the rectum. The 

 auricles are thin- walled conical sacs, the bases of the flattened 

 cones being applied to the sides of the pericardial chamber, 

 and their apices communicating with the median ventricle. 

 The aperture of communication between the auricle and 

 ventricle is a longitudinally elongated valvular slit. If the 

 rectum, a little anterior to the ventricle, be severed, and the 



