98 THE FRESHWATER MUSSEL 



with a razor into a series of slices about a. quarter of an inch 

 thick ; arrange the sections in order in a dissecting dish under 

 water ; examine and draw them. 



The most instructive sections are those passing, (1) through 

 the renal and reproductive apertures ; (2) through the middle 

 of the ventricle ; (3) through the posterior adductor. The 

 chief features of these sections are as follows. 



A. Transverse Section through the Benal and Beproductive 

 Apertures. 



The section passes through the widest part of the body, a 

 little way behind the umbo. 



1. The mantle-folds arise very close to the dorsal surface, 



one on each Side of the pericardial cavity. The 

 folded margin which secretes the two outer layers 

 of the shell should be noticed. 



2. The visceral mass is large and laterally compressed, 



and lies in the middle of the section between the two 

 mantle-lobes. The chief part of its substance is 

 made up of the lobes of the ovary, or testis. The 

 ventral edge of the visceral mass is produced into the 

 wedge-shaped muscular foot. 



3. The alimentary canal. The coils of the intestine are 



seen in the visceral mass, imbedded in the genera- 

 tive gland, and cut across about half a dozen times. 

 Lying in the pericardial cavity in the median line 

 immediately beneath the dorsal surface is the rectum, 

 in which the typhlosole or fold of its ventral surface 

 is well seen. 



4. The gills lie in the dorsal half of the mantle-cavity, 



between the visceral mass and the mantle. The 

 attachments of their lamellse to the body-wall are 

 weU seen. The inner lamella of each inner gill is 

 attached to the base of the visceral mass. 



At the base of each gill is the supra-branchial 

 canal, along which the respiratory stream of water 

 flows backwards to the cloacal cavity. 



