FRESH-WATER MUSSELS. 



143 



begin their breeding activities on falling temperatures of one season, but discharge the 

 glochidia on rising temperatures of the following season. 



Several experiments have shown that the glochidia taken from long-term breeders 

 in the fall of the year may be successfully infected upon fish and that the young mussels 

 will undergo development. It appears, however, that these "green" or newly formed 

 glochidia require a longer period of parasitism than those which have been nursed by the 

 parent through the winter season (Corwin, 1920). 



The origin and purpose of the retention of glochidia during the winter season re- 

 mains a mystery. This may be an instance of nature's remarkable adaptations, per- 

 mitting the development of the egg to occur during the warmer months of summer, and 

 the glochidia to be discharged for attachment upon fish in the spring when there is a 

 general tendency toward an upstream movement of fishes. It is distinctly interesting 

 to note that the long-term breeders (mucket, sand-shells, etc.), as a general rule are 

 mussels of much more rapid growth than the short-term breeders (niggerhead, pimple- 

 back, etc.), although the young of the former are delayed for nearly a year in becoming 

 attached to fish and completing their metamorphosis. 



It is important to point out one fact which is clearly established by data in Table 

 15, page 141. There is no month of the year in which a considerable number of commer- 

 cial mussels are not gravid with glochidia. This fact deser^^es careful consideration in 

 connection with measures of conservation, since it makes impracticable the protection 

 of mussels by "closed seasons" of months based upon the times of breeding. 



GLOCHIDIUM. 



The lar\^al mussel or glochidium, when completely developed and ready to emerge 

 from the egg membrane and before attaching itself to 

 a fish, has apparently an extremely simple organization. 

 The soft mass of flesh possesses neither gills nor foot nor 

 other developed organ characteristic of the adult mussel, 

 but it bears a thin shell composed of two parts which 

 are much like the bowls of tiny spoons hinged together 

 at the top (text fig. 8). The two parts or valves of the 

 shell can be drawn together by a single adductor muscle, 

 but, when the muscle is relaxed, they gape widely apart 

 as shown in the illustration. There are also on the 

 inner surface of each side of the body several pairs of 

 "sensory" cells with hairlike projections. It has been 

 assumed that the cells were sensory in function, and 

 recently L. B. Arey, working at the Fairport station, 

 determined after detailed experiments upon several 

 species of Lampsilis and Proptera that there is a well- 

 developed sense of touch centralized in the hair cells. He regards the tactile response 

 as entirely adequate to insure attachment of the glochidium. 



In at least three genera of American mussels (several species of Unio, Anodonta, 

 and Quadrula) the glochidium possesses a peculiar larval thread of uncertain signifi- 

 cance (text fig. 8). This thread, so generally mentioned in textbooks based upon studies 

 of European mussels, is not found on the great majority of American species. We 



Fig. S. — Glochidium of Quadrula heros with 

 gaping valves, seen from a side view. 

 The lar\*al thread (/. /.) is seen between 

 the valves. Inner and outer sensory hair 

 cells (i. h. c.) are visible on each valve. 



