270 



LOWER INVERTEBRATES. 



Fia. 286. — Glochidium of 

 Unio, still within the egg 

 iiiembraiie; 6, byssut*; m, 

 adductor muscle; s, sense 

 organs; t, teeth; v, velum. 



seem to be founded on individual variation. The shells are usually long, equivalve, 

 and are covered externally with a smooth, thick epidermis, which is of various shades 

 of brown, olive, and green. Internally the shell is pearly ; sometimes white, at others 

 dark pink or almost purple. The hinge ligament is large and external, and the ante- 

 rior hinge teeth are large and thick, the posterior compressed and laminar ; or all may 

 be wanting. Both adductor muscles are present. The mantle edges are united 

 posteriorly so as to form a rudimentary siphon. The foot is very large and tongue- 

 shaped, and in the adult occasionally secretes a byssus. 



One of the most interesting features connected with the fresh-water mussels is found 

 in their development. The eggs are carried in brood pouches 

 between the lamellae of the outer gills, and there undergo their 

 early development, the details of which need not be described 

 here. Eventually a shell is formed, which soon divides into two 

 valves united by a straight hinge. Soon at the fore extremities 

 of each valve a strong beak-like process is developed, and a little 

 later a byssus is secreted. Next, peculiar sense organs are 

 formed on the inner surface of the mantle, the function of 

 which 1!^, doubtless, to ascertain the presence of fishes. Now 

 the young is in the condition known as a Glochidium, and when 

 the mother is under natural conditions or placed in a tank witli 

 fish, the young are expelled from the brood pouch and almost 

 immediately attach themselves to some submerged object by means of the byssus. If 

 no fishes be present, the mother will retain the young for a long time after the glochi- 

 dium stage has been reached. As soon as an opportunity is afforded, the glochidia 

 attach themselves to the gills, fins, or other parts of a fish, by closing the valves of the 

 shell and holding on by the spiny beaks mentioned above. Here they undergo an 

 extensive metamorphosi.s, in which the single adductor muscle disappears and is 

 replaced by the two of the adult, the gills appear, the sense organs atrophy, and, 

 according to one author, the shell and o\en the mantle lobes are formed anew. 

 Finally, when all the organs except those concerned in reproduction are formed, the 

 embryo quits its host and sinks to the bottom, where \)y a regular growth it attains 

 the adult condition. This parasitic or semi-parasitic life is very unusual among the 

 Mollusca. 



The Unionidse live in still water or in running streams, usually about half buried 

 in the mud. They do not, however, lead sedentary lives, but bur- 

 row or plough along the bottom, the part of the shell which con- 

 tains the siphons being above the surface of the gravel or mud. 

 According to Dr. Lea one species occurring in the United States 

 {Margaritana dehiscens) buries itself completely in the mud and 

 di'aws its water and food through a tube, as do so many of the 

 siphonated marine forms. 



In the typical genus Unio, the hinge is provided with one or 

 two anterior and one posterior teeth in one valve, and two anterior 

 and two posterior in the other. The shell is smooth or ribbed or 

 even spined, and the variations in shape are extraordinary. In 

 some of the species the valves become soldered together at the FiG.287.-c/mosj;i«o™s. 

 hinge, so that motion would be impossible were it not for the fact that a fractui-e takes 

 place near the line of junction, so that one valve bears two wings while the other has 



