Mar. 1 6, 1888.] 



SCIENTIFIC NEWS. 



25-r 



Natural Itfjstori). 



FRESHWATER MUSSELS. 



Like most common natural history objects mussels are 

 of great interest from many points of view. Their large 

 size at once attracts attention, and excites some amount 

 of surprise ; the shells sometimes measuring as much as 

 seven or eight inches, by five inches. This is generally 

 where the conditions are favourable, and where there 

 is plenty of food. The freshwater or river-mussel can 

 scarcely be said, however, to select food. The gills, which 

 are in incessant motion, create a current, which draws into 

 the mouth any matter, whether animal, vegetable, or inor- 

 ganic, that may be in the vicinity, and are aided by the 

 action of the tentacles. Decomposed animal and vegetable 

 matters are the chief food of these molluscs ; and in ponds, 

 where there is plenty of this kind of food, in the 

 shape of dead cats, dogs, or fish, and where the water is 

 seldom disturbed, they become of a large size. During 

 the summer months they are active, and crawl about the 



the water. The inside portion from its nacreous or 

 pearly condition is always attractive in appearance, this 

 peculiar lustre being a good example of the interference 

 of rays of light reflected from different surfaces, which is 

 known by the name of diffraction. 



One species of freshwater mussel is called the pearl- 

 mussel, and has been known as such for centuries. 

 They yield fine large pearls, and at one time the pearl 

 fisheries of Britain were a source of considerable revenue 

 to their owners, it having been stated that the value of 

 the pearls from the River Tay, in Perthshire, during the 

 3'ears 1761 to 1764 reached ;,^io,ooo. 



Pearls are formed by layers of this nacre round foreign 

 bodies, which have been introduced either through 

 accident or design. The Chinese have long taken advan- 

 tage of this irritability of the mussel tending to form 

 pearls around foreign substances, and introduce various 

 objects which they wish to become coated with pearl. 

 In the museum of the Royal College of Surgeons may 

 be seen the shell of a freshwater mussel from the neigh- 

 bourhood of Ningpo, into which twenty-three plates of a 

 white metal, each stamped with a figure of Buddha, 



Freshwater Mussel. 

 a. Exhalent syphon, d. 



mud in which they live, leaving a more or less marked 

 groove after them, locomotion being effected by a large 

 fleshy somewhat triangular foot (see fig. i). During 

 the winter they bury themselves in the mud, and hyber- 

 nate ; even in the summer they do the same, should 

 an unusual drought occur, and the mud be left dry. 



As will be seen from the illustration, the animal is 

 protected, like the marine mussels, by a shell, but of an 

 entirely different shape. The shell of the freshwater 

 mussel varies very much, according to its surroundings. 

 There are six species living in English waters, all of 

 which are very variable, the most common being the 

 one illustrated. 



The shells of some of them were made use of by 

 Dutch painters for holding their colours, and at the pre- 

 sent day they may be seen in the windows of colour shops, 

 containing gold and silver for illuminating purposes. 



If the shell be examined it will be noticed that the 

 outside has a black skin, or periostracum covering it, 

 while the inside is pearly. The black skin is generally 

 supposed to protect the shell against being worn away 

 or dissolved by the acids or gases sometimes present in 



A.XODONTA CyGNEA. 

 Labial palps, c. Foot. 



have been introduced, and have afterwards been coated 

 with a layer of nacre. 



All the freshwater mussels develop pearls, but all are 

 not valuable, being irregular, or faulty; their prevailing 

 colour is white, but black, green, brown, and flesh- 

 coloured, or pink ones, are occasionally found, the latter, 

 when large and well-shaped, being the most valuable. 



The development of the freshwater mussel is one ot 

 the chief points of interest. The eggs are generally laid 

 in the later summer or autumn months, but they do not 

 leave the mother-animal, being hatched in the branchiae 

 or gills. The development within the egg is introduced 

 by an unequal segmentation, and results in the forma- 

 tion of a blastosphere with a large segmentation cavity. 

 The embryo, which is partially ciliated, and rotates with- 

 in the egg-membranes, soon acquires a ciliated velum 

 and shell-gland. The shell is formed as a continuous 

 saddle-shaped plate on the dorsal surface. From this 

 plate the two valves are subsequently differentiated ; on 

 the dorsal surface they meet v/ith a straight hinge-line. 

 Each valve is at first rounded, but subsequently becomes 

 triangular with the hinge-hne as a base. At a later 



