THE DEVELOPMENT OF LAMEIXIBRANCHS. 311 



instead of being loosely attached to each other at intervals, 

 as in the inter-tentacular junctions of Mytilus, are fused 

 together to form a continuous lamella. 



XX\^II. THE DEVELOPMENT OF LA3IELLI. 

 BRANCHS. 



Among the Unionidfe the spawning season is very short, 

 and the early changes of the egg take place so rapidly 

 that it is rather difficult to find them for study ; and as 

 the later stages in the fresh-water forms are very aber- 

 rant, it is best for the beginner to study one of the more 

 typical salt-water forms. The spawning season is short 

 with them also, but it comes at different times in different 

 species, and the examination of a number of forms will 

 usually result in the discovery of sexually mature speci- 

 mens of some species at almost any time during the sum- 

 mer months. When the reproductive elements are ripe, 

 or nearly so, the abdomen is more or less distended by the 

 reproductive organs, and the student can therefore judge 

 what form to select for experiment. The method of 

 artificial fertilization, which is described in Section XIV., 

 is to be employed, but it is much more difficult to obtain 

 perfectly ripe reproductive elements than it is with the 

 sea-urchin ; and the student should not be discouraged by 

 repeated failures. 



I. The FertUization of the Eggs. 



Having obtained and opened a number of specimens of 

 a species which seems favorable, examine the contents of 

 the reproductive organs in the following manner, in order 

 to find the most perfectly ripe individuals. 



If the point of a knife be pushed into the reproductive 

 organ a milk-like fluid will ooze out of the cut, and a little 



