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 lamellae 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 lamellae 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, i.e., those belonging 



