236 ZOOLOGY. 



Left mantle flap. How related to the right? to the shell? 



Siphons : modifications of the posterior margins of the 

 mantle. (These will be conspicuous or rudimentary in ac- 

 cordance with the species studied.) Number? 



Adductor muscles of the valves ; number and position. 



Mantle cavity. Separate the right and left mantle lobes 

 along the ventral margin, except in the region of the siphon, 

 and fold back the left. Where is it attached to the body? 

 The ventral or incurrent siphon opens into the branchial 

 chamber, the dorsal or excurrent into a smaller dorsal cham- 

 ber, the cloacal. Verify and sketch. 



Gill plates or sheets; number and attachment. Are they 

 symmetrical on the two sides? The eggs and developing em- 

 bryos may be found in the outer gill cavity at favorable times. 

 (A special study and report may be profitably made by some 

 student on the structure of the gills as shown by a hand lens 

 and the low power of the microscope. A bit of the living 

 gill from a fresh specimen should be examined.) 



Abdomen, the soft, fleshy mass between the pairs of gills, 

 which terminates in a more solid part, 



Foot : position and form ? 



Mouth and labial palps; at the anterior end and just below 

 the adductor muscle. How many palps? 



(It is to be remembered that all the structures examined 

 thus far are external organs. The body wall has not been 

 penetrated at all. If it is the plan to study the anatomy more 

 closely, the following are the chief sets of organs deserving 

 attention. ) 



4. Other systems of organs. 



Circulatory system. Open the pericardial cavity, just beneath the hinge 

 and a little posterior thereto, find the 



Heart: auricles and ventricle. In a fresh preparation the contrac- 

 tions of the heart may be observed. 

 Vessels: one passes in each direction, but they are not easily seen 



without injecting. 



The intestine passes through the ventricle without open communica- 

 tion with it. 



