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 lamellae 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 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 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 lamellae. Mount a piece of a lamella in water and 



