REPRODUCTION AND ARTIFICIAL PROPAGATION OF FRESH-WATER MUSSELS. II 5 



Observations on the passage of the eggs from the ovaries to the gills are extremelv 

 meager, and further information is needed concerning the factors involved in directing 

 the stream of eggs from the openings of the oviducts to their final resting place in the 

 water tubes of those regions of the gills which function as brood chambers. We owe 

 to Latter (1891, 1904) the most detailed account of this process which we have, and, in 

 lieu of any direct observations of our own on the subject, we may quote his interesting 

 description (1891) which is based upon AnoJonta: 



If a female be taken from the shell at this season (the spawning season) the eggs maj' be seen through 

 the transparent wall of the oviduct passing singly, but in a steady stream, to the genital aperture. Their 

 motion is due partly to " labour contractions " of the intrinsic muscles of the foot and partly to the cili- 

 ated lining of the oviduct itself. One by one the eggs issue from the genital aperture, whence they are 

 conveyed backwards by the abundant cilia which clothe the external surface of the nephridium. Along 

 the middle line of this surface there is a belt of especially long cilia which appear to be devoted to the 

 transit of the eggs; those dorsal and ventral to the belt work obliquely so as to keep the eggs in contact 

 with it. It is probable that the free dorsal border of the inner lamella of the inner gill plate is, under 

 normal conditions, applied to the visceral mass in tliis region so as to inclose a temporary tube, one of 

 whose walls is formed by the above-mentioned belt of specialized cilia." In the course of about 50 

 seconds an egg is thus swept back to the slit between the protractor muscle of the shell and the point of 

 fusion of the right and left inner gill lamellse; here they meet the stream of ova from the other side of 

 the body and so reach the exhalent current and the cloaca. 



The process goes on for some 10 days or more in each individual and the number of eggs is immense 

 * * * probably half a million may be taken as a fair average. On reaching the cloaca * » * 

 their direction is reversed and they pass forward into the cavities of the right and left gill plates, which 

 serve as brood pouches. The method by which this change of direction is accomplished is not quite 

 clear. * * * l have, however, observed on several occasions a violent and sudden reversion of the 

 water currents such as would certainly be fully capable of carrying the eggs forward and into the latticed 

 recesses of the outer gills. This reversion is caused by the animal, firstly, closing all the ventral border 

 of the shell by means of the free edges of the mantle assisted by the flexible, uncalcificd rim of periostra- 

 cum and leaving the siphons alone open, and, secondly, relaxing the adductor muscles so as to allow the 

 elastic ligament to make the valves gape apart. These actions cause the hydrostatic pressure within 

 the shell to be less than that of the water without and consequently there ensues a rush of water into the 

 shell through the open siphons. The whole procedure may be likened to a gulp and is achieved by 

 precisely similar physical forces. 



This may possibly be the correct interpretation of the process, but additional 

 observations and experiments should be made for verification. Latter also attempts 

 to account for the fact that the eggs in Anodonta pass into the outer gill and not into 

 the inner, but his explanation is unsatisfactory and inadeciuate. It would be a matter 

 of the greatest interest to discover the mechanism which directs the eggs in the different 

 types of the marsupium into certain water tubes of the gills and not into others. 

 Special structural modifications must be correlated with the particular type as the 

 fundamental cause of these differences, and a very pretty problem is here presented in 

 the determination of such correlations. Since in the genus Ouadrula all four gills 



a It is to be remembered that this description is based upon the conditions as they occur in Anodonta, in which the inner 

 lamella of the inner gill is not fused to the visceral mass, and the inner suprabranchial chamber is consequently freely open to the 

 mantle chamber; in those forms, however, in which this lamella is fused for a part or all of its length, the eggs are received into 

 the anterior end of the inner suprabranchial chamber, into which the genital apertures open directly, and pass back through this 

 chamber to the cloaca. 



