A HARD-SHELL CLAM 107 
body-wall a dark-colored mass, the liver, back of which are 
the yellowish reproductive gland and the dark-colored organ of 
Keber. Back of the latter is the pericardium, within which is the 
heart. Beneath the heart and in front of the posterior adductor 
muscle is the dark-colored kidney. Passing through the peri- 
cardium and the heart and above the posterior adductor muscle 
to the cloaca will be seen the rectum. It ends with the anus near 
the hinder surface of the muscle. Open the cloacal chamber 
by a slit in the side of its siphon and find the anus. 
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 are, by way of origin, but a single 
organ, which is called the ctenidium. The clam is thus provided 
with a single pair of ctenidia, which are homologous to those of 
the squid and of snails. Each gill consists of a pair of plates 
or lamelle united at their lower edges and open above, and fur- 
ther joined by vertical or dorso-ventral cross-partitions, the inter- 
lamellar 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 supra- 
branchial passage. This is a wide canal running along the base 
of each gill to the cloacal chamber. The course of the supra- 
branchial passage of the outer gill has already been noted. In 
order to observe that of the inner gill, lift up both gills; the 
inner suprabranchial passage will be seen at the base of the 
inner gill. Probe from the cloacal chamber into it. Notice 
that back of the visceral mass the two inner suprabranchial 
passages coalesce and form a single passage. 
Study the finer structure of the gills. Place a gill on a 
glass slide in a little water and with forceps and knife carefully 
separate the lamella. Mount a piece of a lamella in water and 
