310 MOLLUSCS. 



accumulate in crowds in the outer gill-plates. The repro- 

 ductive organs are branched and large; there are no 

 accessory structures; on each side a genital aperture lies 

 under that of the ureter. 



Autumn and winter months seem to be the favourite 

 periods of reproduction. The ova are squeezed out of the 

 foot, and appear to be moved to the exhalent region, whence 

 however they do not escape, but are crowded backward till 

 they pass into the cavity of the outer gill-plate. At some 

 stage they are fertilised by spermatozoa drawn in by the 

 water-currents, though it is difficult to believe that this is 

 entirely a matter of chance. Development takes place in 

 the gill-cavity, which is often much distended with larvas. 



Development and Life-History. — The egg-cell is surrounded 

 by a vitelline membrane, and is attached to the wall of the 

 ovary by a tiny stalk, the insertion of which is marked on 

 the liberated ovum by an aperture or micropyle, through 

 which a spermatozoon may eventually enter. 



The segmentation of the fertilised ovum is total but 

 unequal. A number of small clear yolkless cells are rapidly 

 divided off from a large yolk-containing portion, which 

 remains for a while unsegmented. Eventually, however, a 

 hollow ball of cells or blastosphere results. 



A gastrula is formed from the blastosphere, by the in- 

 dimpling of the lower, larger, yolk-containing cells, so that 

 they are surrounded by the clearer small cells. As this 

 gastrula is a-making, two large cells belonging to the lower 

 series are set free into the cavity of the blastosphere. 



The outer clear cells are ectodermic, and will form epi- 

 dermis, nervous system, etc., the invaginated heavier cells 

 form the endoderm which lines the gut; the two cells 

 liberated into the cavity begin the median stratum or meso- 

 derm, from which arise muscles, connective-tissue, etc. 



If the development of Anodonta were like that of most of 

 the bivalves whose development we know, its gastrula 

 would grow into a trochosphere, and that would become 

 a veliger. Instead of this, however, a peculiar larva re- 

 sults which is specially adapted to the risks of life in fresh 

 water. 



In the winter months these peculiar larvae are abundant 

 in the cavity of the outer gill-plate of the female mussels, a 



