138 BULLETIN OF THE BUREAU OF FISHERIES. 



inches) in length and of an estimated age of 8 years. Females of the Lake Pepin mucket, 



LampsUis luteola, reared at the U. S. Fisheries Biological Station, Fairport, Iowa, were 



found \vith mature glochidia in the third season of growth, a period of slightly more 



than two years after dropping from the fish. Undoubtedly not all species breed at 



such an early age, and it perhaps takes the heavier Quadnilas 6 or 8 3'ears to reach the 



breeding age. 



OVULATION AND FERTILIZATION. 



• With a few exceptions," the sexes are separate in American species of fresh-water 

 mussels. The discharge of eggs (ovulation) has been observ^ed in some instances (Latter, 

 1891; Ortmann, 191 1, p. 298; and Howard, 1914, p. 35). The eggs pass from the 

 ovaries by way of the oviduct, through the small genital aperture into the cloaca and 

 suprabranchial chambers, and then into the portions of the gills which are to serve as 

 brood pouches. The sperm which has been thrown out into the water by one or more 

 male mussels, doubtless those in the near vicinity of the female, is taken in by the female 

 with the respiratory current, but whether the eggs are fertilized while on the way to 

 the brood pouches or after reaching them is unknown, since the process of fertilization 

 in nature has never been observed. We have no clue either as to the nature of the 

 stimulus which may excite ovulation or as to how it may be timed so as to take place 

 when a supply of living sperm is available in the water for the fertilization of the eggs. 

 Certain it is that the eggs are usually fertilized, although in the brood pouches of any 

 gravid mussel that may be examined there are found a good many eggs that have failed 

 to develop, presumably because they have escaped fertilization. 



The discharge of sperm in great quantities may not infrequently be observed when 

 male mussels are retained in aquaria. The writers have observed in a large tank at the 

 Fairport station a male mussel discharging sperm. During the process it traveled exten- 

 sively over the bottom, leaving in the sand a long winding furrow which was filled with 

 a white cloud of sperm. Perhaps the discharge of sperm and its introduction with the 

 respiratory current into the female constitute the exciting cause of ovulation. Exper- 

 iments are clearly wanted to determine this question. The arrangement of the eggs 

 in the several chambers of the brood pouches varies according to the character of the 

 pouch, and will therefore be more conveniently described in the following section. 



BROOD POUCHES OR MARSUPIA. 



The gills of mussels, as of other lamellibranch mollusks, are thin flaps that hang 

 like curtains from each side of the body, a pair on each side. As explained in another 

 place (p. 175) each gill, thin as it may appear, is really a double structure, or more cor- 

 rectly is a sheet folded upon itself just as a map, larger than the page of a book in which 

 it is bound, is folded on itself. There is this difi'erence; the map may be unfolded at 

 will, but the gill may not, because the two sections are attached together by many par- 

 allel partitions which divide the narrow space between the sheets into a lot of long 

 slender tubes. It is into these tubes that the eggs are deposited, and when filled with 

 eggs or glochidia the several tubes are greatly distended (text fig. 7). The entire gills 

 or the parts of the gills bearing the eggs then appear not as thin sheets but as thick 



" The known exceptions are. occasionally. Quadruia rubifinosa and pyramidata. and LampsUis parva, and. usually, Anodonla 

 imbecUlis and hcnryana (Sterki, 1S98). and Symphytwta compressa and -viridu (Ortmann. 1911. p. 308). 



