176 BULLETIN OF THE BUREAU OF FISHERIES. 



are covered by cilia or minute paddles, the combined action of which forms a wonderful 

 mechanism for conveying the food from any point of the gill surface into the funnel- 

 shaped mouth. The detailed working of this mechanism and the places and means of 

 "switching off" undesirable matter form too complex a subject to be treated in this 

 paper. (See Allen, 1914, and Kellogg, 1915.) 



The course of the water is better understood after observing the mode of attachment 

 of the gills. The outer lamella of the outer gill is attached to the mantle throughout 

 its entire length, while its inner lamella and the outer lamella of the inner gill are attached 

 together to the body. There is thus above each gill a small suprabranchial chamber 

 just above the water tubes. Behind the body or visceral mass, however, the inner 

 lamella of the right and left inner gills are attached together, and there is, therefore, 

 a single large chamber above the four gills — the cloaca or exhalent chamber. The 

 water, after passing through the pores of the gill surface, makes its course up the water 

 tubes and backward by the suprabranchial chamber into the cloaca, to be passed thence 

 out of the shell." 



It will be understood that the eggs and young borne in the water tubes of the gills, 

 which become marsupial pockets, are most favorably located for respiration, being 

 situated, as it were, in the respiratory current of the mother. There is, among the 

 various species of the Unionidae, great variation in the extent to which the gills are 

 employed as marsupia (p. 139). In certain species the water tubes of all four gills are 

 filled with eggs, in others only those of the outer gills receive the eggs, while in still 

 others a portion of each outer gill is set apart as a marsupium. This may be the posterior 

 half, the posterior third, or a few water tubes in the middle. 



It is largely because of the great significance of the gills with their remarkably 

 diverse functions of food collection, respiration, and gestation that the modifications 

 both in the external form and in the histologic structure of the gills are important and 

 ser\'e so well as a basis of classification. Generally speaking, species in which all four 

 gills serve as marsupia are considered lower or more primitive forms. Those in which 

 the marsupia are most highly specialized are regarded as most highly developed. 



° The effect of the gills in filtering the water is made clear when one fills two jars with turbid river water after placing in each 

 sufficient sand for a mussel to become embedded. If one or two mussels are placed in one of these jars, the water will become 

 clear in a comparatively short time. 



