CLAM 1 



1. The shell of a Clam consists of right and left halves (valves) which 

 are hinged together along the dorsal surface. Concentric lines of growth 

 are visible on the shell, which radiate from a dorsal portion, or umbo, 

 where growth of the shell started. Draw the external surface of the left 

 valve. 



2. Carefully remove the left valve from your specimen, noting the 

 structure of the hinge and the attachments of the anterior and the 

 posterior adductor muscles to the interior of the shell. It is necessary 

 to cut both of these muscles before removing the valve. Note that the 

 internal organs are enclosed by a membranous mantle which lines both 

 valves of the shell. The space between the two halves of the mantle, in 

 which the organs he, is known as the mantle cavity. Turn back the 

 left half of the mantle, cutting a little at each end, and thus expose the 

 structures in the mantle cavity as follows : (a) the large dorsal visceral 

 mass, from which (ft) the ventral muscular foot extends ; (c) the delicate, 

 striated gills; and (d), at the posterior end of the mantle, the dorsal 

 (exhalent) and the ventral (inhalent) siphons. 



3. Below the anterior adductor muscle, note the two pairs of palps, 

 between which the mouth is situated. At the posterior end of the body 

 find the anus, which opens into the exhalent siphon, dorsal and slightly 

 posterior to the posterior adductor muscle. Trace the intestine ante- 

 riorly from the anus to the heart. The latter lies in a median dorsal 

 position enclosed in the thin, delicate pericardium. The heart consists 

 of a single ventricle through which the intestine passes, and two small, 

 thin-walled auricles lying laterally and below and attached to the peri- 

 cardium. The auricles are easily destroyed in the dissection. Locate 

 the anterior aorta continuing anteriorly from the heart and the posterior 

 aorta continuing posteriorly along the intestine. On the latter note, in 

 some species, the bulbus arteriosus a short distance back of the heart. 



4. Make an enlarged drawing showing the internal structure of the 

 Clam as observed. 



5. Examine the external structure of various other types of Molluscs, 

 such as (a) the common garden Slug which lacks an external shell, (ft) a 

 Snail with a spirally-coiled shell, and (c) a free swimming Squid with 

 prominent head region. 



i B. pp. 139-148. 

 305 



