88 PHYLUM MOLLUSCA 



Carefully remove the left mantle lobe after cutting it with fine 

 scissors at its line of attachment, beginning at the forward end. 

 Cut ofT the siphonal muscle, leaving the siphons in position. 

 Place the anim^al in water and study the arrangem^ent of the 

 organs. Observe the position of the gills ; note in front of them 

 two triangular flaps, the oral palps ; in the median line between 

 the two pairs of oral palps is the mouth ; find it. Along the base 

 of the gills note an elongated passage leading posteriorly to the 

 cloacal chamber ; this is the suprabranchial passage of the outer 

 gill. Blow into this passage at its hinder end in the cloacal 

 chamber with a blowpipe, or probe it. 



Observe again the siphonal region. Note again the short septum 

 which separates the branchial from the cloacal chamber, and the 

 opening between it and the visceral mass ; probe this opening. 

 Just beneath the umbo will be seen through the semitransparent 

 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 pericardium 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 sur- 

 face 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 men- 

 tioned. Carefully label all. 



The Respiratory System. The gills have already been noticed. 

 The two gills on each side are in 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 further joined by 

 vertical or dorsoventral cross partitions, the interlamellar par- 

 titions. The space between the lamellae is thus divided into 

 parallel, vertical chambers, the water tubes, which run from the 



