MOLLUSCA 243 



Mouth. 



Respiratory orifice, position. Under what circumstances 

 seen? 



3. Shell (secure empty ones). Make sketches of the shell 

 and identify the structures referred to in the following terms: 

 apex, aperture, lip, spire, whorl, suture, columella. (See 

 Fig. no). 



How would you describe the direction of the spiral? How 

 many whorls? Have the young and old the same number? 

 Can you detect lines of growth ? 



4. Soft Parts. These may be removed by dropping the animal suddenly into 

 hot water, and then gradually twisting the soft portion from the shell. It will 

 scarcely repay the trouble to do more than re-identify the following parts: mouth, 

 respiratory orifice, mantle and mantle chamber, and collar (a portion of the 

 mantle). The spiral is occupied by the digestive tract, its glands, the reproduc- 

 tive bodies, etc. 



5. Development. Examine the stems of plants and the sides of the vessel 

 in which snails have been kept for some days for gelatinous capsules of eggs. They 

 are almost transparent and the eggs may be easily located. What seems to be 

 the value of the gelatine? Number and arrangement of the eggs? What is the 

 shape of the eggs? Get the earliest stages possible, and watch day by day at 

 short intervals, or compare capsules of different ages. If care is taken, some idea 

 of the early segmentation stages may be obtained. Lookfortheblastula: are the 

 cells of the same size? Do you find a gastrula? What are the first signs you find 

 of differentiation of parts? Look for different stages of the later development. 

 It will not be profitable to try to follow the changes in detail. 



282. A very valuable laboratory exercise may be had by comparing large 

 numbers of shells of a single species, found under varying conditions. Compare 

 as to shape, markings, etc., and see whether there are individuals connecting your 

 extreme groups. The land snail (Helix, Fig. 121) is favorable for such study. 

 Helix may be substituted for Limnaa. 



283. The Squid. The teacher should at least have a few specimens of the 

 Squid, from which the pupils may be required to get some idea of the general form. 

 Drawings should be made, showing all external features. 



Note particularly: 



Head: tentacles, number, comparative length; suckers on the inner surface, 



arrangement of suckers. 

 Eyes: number, size, position. 



Olfactory organs opening beneath folds of skin behind the eyes. 

 Neck. 



Body: general shape. It is surrounded by the 

 Mantle; note the fin expansions at the posterior end. Where are the 



attachments of the mantle to the body? 

 Siphon; how related to the mantle cavity? 



