358 GHiMAN A. DREW. 



formation of the third lobe the plate is extended into it. In 

 this way, as new lobes are formed, the chitinous plate 

 is extended into each, and continues to be connected 

 throughout the length of the gill. As the lobes grow to 

 form filaments, the chitinous plates extend with them, and 

 each becomes trough-shaped with the open side of the trough 

 directed away from the corresponding lobe of the mantle. 

 Later the free edges of the trough are brought near to- 

 gether, and the support in each filament practically assumes 

 the form of a tube that extends out nearly to the tip of each 

 filament. The tubes that support the different filaments are 

 united at their bases, so the chitinous support is continuous 

 throughout the gill. 



As the lobes elongate to form filaments, the cilia on each 

 becomes restricted, so that the side that is turned away from 

 the lobe of the mantle to which it is attached becomes quite 

 free from them. On the remaining sides the cilia are long 

 and powerful. 



About the time that the fourth division of the gill is 

 formed the mantle begins to thicken at the bases of the fila- 

 ments, between them and the shell (fig. 45). These thicken- 

 ings are generally opposite the bases of the filaments, and 

 connected with them, but as there are sometimes more plates 

 on one side of the giW of the adult animal than on the other, 

 the thickenings are probably not always formed in this 

 position. 



They represent the beginnings of the outer plates of the 

 gill. The filaments, at the bases of which these thickenings 

 are formed, form the inner plates of the gill. 



For a considerable time the outer plates remain much 

 smaller than the inner plates, and they never quite equal 

 them in size (fig. 53). As the outer plates of the gill are 

 formed, the chitinous support is carried out into them as 

 branches from the portion that runs lengthwise of the gill. 



1 Tbe term gill is for couvenience applied to the respiratory organ on one 

 side of tlie animal, although writers agree that it probably corresponds to tne 

 two chills found on each side of most Lamellibranchs. 



