A FRESHWATER MUSSEL 93 
pericardium and the heart, then extending above the posterior 
adductor muscle to the cloaca, where it ends with the anus. Cut 
open the cloacal chamber by a slit in the side of its siphon. 
Find the hinder end of the rectum and the anus. Note just 
beneath the muscle a canal which accompanies the base of the 
gill forward. This is the suprabranchial passage of the outer gill; 
it runs posteriorly to the cloacal chamber. Blow into it with a 
blow-pipe, also probe it from behind. 
Exercise 5. Draw a semidiagrammatic view of the animal lying 
in the right-hand valve of the shell, representing the organs 
above mentioned. Carefully label all. 
The respiratory system. The gills have already been noticed. 
The two gills on each side of the visceral mass are, by way of 
origin, but a single organ, which is called the ctenidium. The 
mussel is thus provided with a single pair of ctenidia, which are 
homologous to those of the squid and of snails. Each gill con- 
sists of a pair of plates or lamelle united at their lower edges 
and open above, and further joined by vertical or dorso-ventral 
cross-partitions, the interlamellar partitions. The space between 
the lamelle is thus divided into parallel, vertical chambers, the 
water-tubes, which run from the bottom to the top of the gill 
and open above into the suprabranchial passage. One of these 
passages runs along the base of each gill, as a wide canal, to 
the cloacal chamber. We have already observed the supra- 
branchial passage of the outer gill. In order to observe that of 
the inner gill, lift up both gills; the inner lamella of the inner 
gill will, in most species of mussels, be seen not to be united 
with the wall of the visceral mass along the hinder portion of 
the foot, but to have a free edge. The long slit-like opening 
thus presented leads into the inner suprabranchial passage. 
Probe it backward to the cloacal chamber. Probe it also from 
the hinder end forward and notice that back of the visceral 
mass the two inner suprabranchial passages, 7.e., those belonging 
