THE LAMELLIBRAXCHIATA. 409 



filaments becoming- as it were doubled upon themselves at 

 their free ends, the reflected portions lying on the outer side 

 of the outer, and on the inner side of the inner, series of 

 primary filaments. But the free, or haemal, ends of the re- 

 flected filaments contract no adhesion either with the mantle 

 on the outer side, or with those of the opposite gill on the 

 inner side. Delicate bands stretch from the primary to the 

 reflected filaments across the interspace which they inclose 

 (Mytilus, Pecten). In most Lamellibranchs the gills are 

 four elongated plates, each of which is in fact a long and 

 narrow pouch, with its open end turned toward the haemal 

 face of the body. Two pouches are situated on each side of 

 the mesosoma ; one of these pouches is internal, the other 

 external. 1 Their walls are united by transverse septa ; they 

 are richly ciliated, and are perforated by numerous apertures. 

 As the outer wall of each pouch is united with the mantle, 

 and the inner with its fellow of the opposite side, behind the 

 foot, the whole branchial apparatus forms a sieve-like parti- 

 tion extended between the mantle and the foot (Fig. 117), 

 and thus divides the pallial cavity into a supra-branchial 

 and an infra-branchial chamber. Inasmuch as the haemal 

 edge of the inner wall of each inner branchial pouch is, for 

 the greater part of its extent, not united with the mesosoma, 

 but only closely applied against the latter, the supra-bran- 

 chial and infra-branchini chambers may communicate by the 

 cleft thus formed, as well as by the apertures in the lamellar 

 walls of the branchial pouches. The anterior part of the 

 supra-branchial chamber is divided into a right and left 

 cavity by the interposition of the mesosoma, on the sides of 

 which the apertures of the renal and generative organs are 

 situated. The products of these organs therefore readily 

 pass into these right and left cavities. The posterior part of 

 the supra-branchial chamber, into which these two lateral 

 divisions open, contains the termination of the rectum, and 

 receives the faeces, as well as the urinary and generative prod- 

 ucts: it is therefore a sort of cloaca. Its external opening is 

 usually termed the anal opening of the mantle cavity. The 

 margins of this opening may be produced into a tube which 

 is termed the anal siphon. In front of the anal, or rather 

 cloacal, opening, the margins of the mantle may be com- 

 pletely disunited. Very frequently, however, they are con- 



1 The external gill-pouch is often smaller than the internal. In species of 

 Lucina, Cytherea, and Tellina, only one gill-pouch, the internal, is present. 



IS 



