ANODONTA 59 



pericardial-cavity and opens at the anus (fig. 12, J3). The 

 stomach and coiled part of the intestine are embedded in a 

 mass of glandular tissue, which consists largely of the numerous 

 lobes of the large digestive glands which pour their secretion 

 by several ducts into the stomach. 



The gills of the fresh-water mussel arc as much concerned 

 with nutrition as with respiration. There is a pair of gill-plates 

 on either side of the body. Their 'anterior attachments have 

 already been noted. From thence they pass obliquely back- 

 wards and downwards beneath the posterior adductor muscle, 

 and project beyond the posterior end of the body, their hinder 

 ends being continued into a short horizontal septum, which is 

 attached to the sides and edges of the mantle, and separates 

 the inhalent from the exhalent aperture. In this manner the 

 mantle cavity is divided into a large branchial cavity below, 

 and a smaller cloacal cavity lying behind and above the pos- 

 terior adductor muscle. The water taken in at the inhalent 

 aperture'oFTTFe mantle has to pass through the passages of the 

 gills before it can reach the cloacal chamber and be discharged 

 through the exhalent aperture. The gill-plates and the passages 

 formed by them are complex structures requiring careful study. 

 Each gill-plate is made up of a number of vertical bars, whose 

 free surfaces are covered by a richly ciliated columnar epi- 

 thelium. The bars are elongate oval in transverse section, and 

 their inner sides are thickened and fused together at irregular 

 intervals so as to form a plate perforated by numerous aper- ; / fci 



tures. Further than this, each gill-bar is bent back at a sharp 

 angle, the bars of the outer gill-plate being bent outwards, and 

 those of the inner gill-plate inwards. The reflected parts of the 

 gill-bars are fused together in the same manner as the vertical 

 descending portions, and thus each gill-plate is made up of an 

 outer and an inner lamella, each lamella being formed by the 

 fusion at irregular intervals of a number of gill-bars lying side 

 by side. The external and internal lamella are also connected 

 by a number of vertical ridges parallel to the gill-bars, and con- 

 taining blocd-vessels. Viewed in transverse section ? the four 

 lamellae of the two gill-plates of one side present the figure of 

 a W, and the upper limbs of the W are attached to the mantle, 

 body-wall, or gill-plate of the opposite sides of the body in the 

 manner shown in the diagrams (fig. 13, A, B, and C). The 

 outer lamella of the outer gill-plate is attached by the whole 



\) 



