The Pond Mttssel. 



63 



mollusc and the " animal." This is, it is hardly necessary to 

 point out, an unfortunate use of language, as it implies in 

 a way that the shell is an adventitious structure, like the case 

 of a caddis-worm. The shell is a secretion of the epidermis 

 of the mussel, just as is the shell of the crayfish. So far the 

 two structures are strictly comparable; and the "fishmonger 

 who broadly classifies marine articles of food into ' fish and 

 shell-fish, is not committing so gross a zoological solecism as 

 might be. The part of the body which underlies and forms 

 the shell is known as the mantle ; it is simply a fold of the 

 body drawn out so as to cover over the foot and gills. The 



LIVER 



HEART 



>>:&<■ CCTLOME. 



ANT ADDR 



/POST" ADD" MUS. 



Fig. 29. — Semi-diagrainmatic representation of a dissection of Anodon. 

 (Partly after T. J. Parker.; 



N.B. — The word anus is placed in cloaca! cavity. The parieto-visceral ganglia lie 

 . below posterior adductor. 



same thing has happened as in the crayfish; there there is 

 also a fold of skin drawn out, which secretes the branchiostegite. 

 At the posterior end of the body the two mantle-flaps become 

 joined so as to divide an upper exhalent orifice from a lower 

 inhalent orifice. The edges of the mantle at the latter are 

 fringed with stiff papilte, or tentacles. When the mantle-flap 

 of one side of the body is removed, the structures shown in 

 Fig. 29 are brought into view. The mantle-flaps, like the gills 

 which are now evident, are dependences of the body itself^ 



