328 ZOOLOGY. 



edge of the inner lamella of each outer gill is united for its 

 whole length with that of the outer lamella of the inner 

 gill, so that in transverse section the two gills of each side 

 show a W- sna P e The outer lamella of each outer gill is 

 attached to the mantle for its whole length, while the 

 inner lamellae of the inner gills are united together in the 

 middle line posteriorly, but separate farther forwards to 

 run right and left of the animal's body, to which they are 

 attached at their front ends. The united edges of the 

 inner lamella of the outer gill and outer lamella of the 

 inner gill are also attached to the side of the body in 

 front. By these various unions of the gill-lamellae, the 

 mantle-chamber becomes incompletely divided into two 

 p. larger ventral or infra-branchial chamber, and a smaller, 

 more dorsal supra-branchial chamber, which latter is 

 further imperfectly divided into four chambers, correspond- 

 ing to the four gills. The former may be open freely 

 by the gaping of the valves, and when they are drawn 

 together communicates with the outer water by the in- 

 halent aperture ; the latter communicates with the exterior 

 by the exhalent aperture. The mouth lies in the infra- 

 branchial chamber, just behind the anterior adductor 

 muscle : the anus, the excretory apertures, and the genital 

 apertures all open into the supra-branchial chamber. The 

 posterior, undivided part of the supra-branchial chamber is 

 often called the cloaca, though quite different from the 

 Vertebrate cloaca. 



We must next realize that each lamella is not really a 

 continuous plate, but is a numerous series of vertical 

 filaments, each supported by two chitinous rods and covered 

 by ciliated epithelium. These filaments are united to one 

 another at very frequent intervals, so as to form a sieve or 

 lattice-work. Besides this, there are connexions between 

 the two constituent lamellae of each gill, and in these 

 inter -lamellar connexions run blood-vessels (fig. 169). These 

 mter-lamellar connexions are much more abundant in the 

 inner gills than in the outer, so that while each outer gill 

 contains a large cavity (in which, in the female, the develop- 

 ing embryos are protected), the cavities of the inner gills 

 are reduced to a spongy condition. 



