72 



EDWARD L. RICE. 





There are several such bridges between the limbs of each fila- 

 ment in a well-grown specimen. The particular filament figured 

 had three interlamellar connections ; but other filaments of the 

 same animal showed four. In Modiola, on the other hand, the 



filament limbs are united 

 by a continuous mem- 

 brane (Fig. 7, A, b) which 

 extends from the point of 

 flexure for more than half 

 the length of the reflexed 

 limb. Sectioning shows 

 that this membrane is 

 composed of a very loose 

 connective tissue with 

 abundant blood spaces. 

 The question immediately 

 arises which of these types 

 of connection is to be 

 considered primitive. On 

 other grounds than gill 

 structure the view was ex- 

 pressed in a former paper 

 ('98) that Modiola is the 

 more primitive form. In 

 that connection the a 

 priori argument was ad- 

 vanced that the Mytilns 

 type of gill filament might 

 be easily derived from the 

 Modiola type, while an in- 

 dependent origin of the 

 interlamellar bridges of 

 Mytilns would be more 

 difficult to conceive. 

 The examination of the 

 development of these bridges confirms this view. Fig. 8 shows, 

 in outline, the tips of five consecutive filaments of the gill of 

 Mytilns. The filaments are arranged in the order of their occur- 



Fig. 7. Filaments of adult gill of (A) 

 Modiola and (B) Mylilus. Only a part of 

 Mytilus filament represented, a, interfilament- 

 ary connection (ciliated disk) ; b, interlamellar 

 connection. 



