166 HOUGHTON, ON THE FKY OF ANODONTA CYGNEA. 



"wliicli readily separate from each other ; these and the well- 

 marked adductor muscles of the valves are about all that can 

 he detected. But if the same experiment is made upon a 

 specimen three weeks after it has become a parasite, it will 

 be found that the animal is of firmer consistency; ciliary 

 action is very distinct, and there can be no doubt that the 

 organs are beginning to assume, by degrees, their "ultimate 

 form ; but it is almost impossible to say definitely what are 

 the various stages of development, owing to the opacity of the 

 closed valves. 



The young Anodonta, at the time of exclusion, keep con- 

 stantly snapping together their valves, reminding one of the 

 somewhat similar action of the bird's heads {avicularia) of 

 some of the marine Polyzoa. This is a most curious spectacle 

 to witness, and its meaning is now explained by the parasitism 

 of the young creature, which, after it has once snapped upon 

 the fin of an unfortunate fish, appears to be quite sta- 

 tionary. In fact, the peculiar barbed nature of the hooks 

 of the Glochidium would appear to render it impossible that 

 the valves of its shell shovild ever reopen, so long as they 

 remained attached to the tissues of the fish. And this seems 

 to be still more probable from the circumstance that there is 

 no muscular provision for the opening of the valves, which 

 depends solely upon the elasticity of the ligament. 



Much remains to be yet investigated, but it was thought 

 advisable to publish the foregoing remarks, as they will doubt- 

 less tend to set inquirers to work on a subject full of curious 

 interest. 



The parts most remarkable in the young Glochidium, 

 when the valves are open, are — first, the curious barbed or 

 serrated hooks (figs. 3, 4, 5, 6) at the margin of the shells, each 

 of which appears to be furnished with a muscle on either side, 

 though it is not easy to determine whether these supposed 

 muscles may not be simply membranous folds. We may 

 also observe the strong adductor muscle by which the valves 

 are closed (figs. 3, 4, 6). Each valve is filled up with 

 a soft, granular-looking mass, representing, perhaps, the 

 rudimentary mantle, and the space near the angle is occupied 

 by a more distinctly granular substance, out of which it is 

 probable the visceral mass is subsequently developed. From 

 the middle of this -vdsceral mass arises a small eminence, from 

 the summit of which proceeds the long, slender byssus-fila- 

 ment, which, whilst the embryo is still within the egg-mem- 

 brane, is coiled up, and fills the greater part of the space 

 between the lateral lobes (fig. 3). After the embryo has 

 escaped from the egg-membrane, and the byssus- filament 



