310 ANIMAL BIOLOGY. [Part II. 



of Mollusca to which Anodon belongs the name of Lamelli- 

 branchiata, lie, as we have seen, on either side of the foot 

 anteriorly, and hang down into the infra-branchial cavity pos- 

 teriorly. Each gill-plate consists of two lamellae, an inner and 

 an outer, or preferably a descending and an ascending lamella, and 

 each lamella may be regarded as arising from the union of a 

 great number of gill-filaments, descending and ascending, which 

 have become fused together into a perforated plate-like structure. 

 Fig. 91 will exhibit the relations of the lamellae. Fig. 91, A., is a 

 section passing through the middle of the ventricle (ve.) and the 

 posterior part of the foot (/.). The gill-filaments take their 

 origin from a gill axis (g. a.), forming in this part of the body a 

 ridge on the body-wall. From this axis there pass downwards 

 two descending filaments, an inner, forming part of the inner 

 lamella of the outer gill, and an outer, forming part of the outer 

 lamella of the inner gill. After proceeding downwards the fila- 

 ment turns sharp round and forms an ascending filament, running 

 parallel with the descending filament. The ascending filament 

 of the outer gill forms part of its outer lamella ; that of the 

 inner gills forms part of its inner lamella. The upper ends of 

 the ascending filaments of the outer gills become united by 

 concrescence along their whole length with the mantle. The 

 upper ends of the ascending filaments of the inner gills are in 

 this region free, thus giving rise to the slit-like communication 

 between the supra-branchial chamber (s. b. c.) and the infra- 

 branchial chamber (i. b. c.). 



Fig. 91, B., is a diagrammatic section taken through a region 

 anterior to that shown in A., nearer the anterior end of the gill- 

 plates. It shows that here the ascending lamina of the inner 

 gill has united by concrescence with the foot. Fig. 91, C., is a 

 similar section taken through the posterior adductor muscle. It 

 shows that here, in the region posterior to the foot, the ascend- 

 ing lamellae of the inner gills have united with each other by 

 concrescence. Fig. 91, D., is taken through the siphonal cavity, 

 behind the posterior adductor. It shows that the gill-axis, 

 which in A. and B. was closely adherent to the body -wall, and in 

 C. was suspended by a suspensory ligament, has here become free. 



