664 ZOOLOGY SECT. 



tubes, one (ex. sph.) smooth-walled, the other (in. sph.) beset with 

 delicate processes or fimbrice. By diffusing particles of carmine 

 or indigo in the water it can be seen that a current is always passing 

 in at the fimbriated tube, hence called the inhalant siphon, and out 

 at the smooth or exhalant siphon. Frequently a semi-transparent, 

 tongue-like body (ft.) is protruded between the valves at the opposite 

 side from the hinge and at the end furthest from the siphons : 

 this is the foot ; by its means the animal is able slowly to plough its 

 way through the sand or mud. When the Mussel is irritated the 

 foot and siphons are withdrawn and the valves tightly closed. 

 In a dead animal, on the other hand, the shell always gapes, and it 

 can then be seen that each valve is lined by the corresponding lobe 

 of the mantle, and that the exhalant siphon is formed by the union 

 of the lobes above and below it and is thus an actual tube ; but 



i? 



FIG. 572. Anodonta cygnea. The entire animal. A, from the left side ; B, from the 

 posterior end. d. p. a. dorsal pallial aperture ; ex. sph. exhalant siphon ; ft. foot ; in. sph. 

 inhalant siphon ; Ig. ligament ; m. mantle ; um. umbo. (After Howes.) 



that the boundary of the inhalant siphon facing the gape of the shell 

 is simply formed by the approximation of the mantle-lobes, so that 

 this tube is a temporary one. 



The hinge of the shell is dorsal, the gape ventral, the end bearing 

 the siphons posterior, the end from which the foot is protruded 

 anterior : hence the valves and mantle-lobes are respectively 

 right and left. 



In a dead and gaping Mussel the general disposition of the parts 

 of the animal is readily seen. The main part of the body lies be- 

 tween the dorsal ends of the valves : it is produced in the middle 

 ventral line into the keel-like foot, and on each side, between 

 the foot and the corresponding mantle-lobe, are two delicate, 

 striated plates, the gills (Figs. 576-578). Thus the whole animal 

 has been compared to a book, the back being represented by the 

 hinge, the covers by the valves, the fly-leaves by the mantle lobes, 

 the first two and the last two pages by the gills, and the remainder 

 of the leaves by the foot. 



The shell. When the body of the mussel is removed from 



