MOLLUSCA — GASTROPODS 24 1 



11. Sketch the entire body removed from the shell. Indicate 

 mantle, siphon, spire, columellar muscle, liver, operculmn, foot. 



12. What is the function of the operculum? 



13. Did the body of the animal occupy the whole or only a 

 portion of the shell ? 



14. How w^as the shell built ? 



15. Are the sexes separate or united in one individual ? 



16. How are the eggs laid? How many at a time? What 

 proportion of these reach maturity ? 



17. Define the protoconch. What does this indicate as to 

 the past history of Busycon ? 



18. How would an internal mold of the sheU be formed? 



General Survey of Class Gastropoda 



Gastropoda are with few exceptions characterized by the 

 possession of a distinct head, creeping foot and single mantle 

 which in most forms secretes a spiral or saucer-shaped shell. 

 The head, except in a few degenerate forms, bears tentacles and 

 eyes. 



Nearly all gastropod shells are more or less coiled, resulting 

 from the more rapid growth of one side of the body and its 

 mantle. It is usually the left side which grows more rapidly 

 than the right, producing a dextral (right-handed) coil. If, 

 when the shell is held apex up with the aperture towards the 

 observer, the aperture is on the right, the coiling is said to be 

 dextral ; if on the left, it is sinistral. 



In addition to this external asymmetry produced by the coiling 

 of the shell, gastropods are characterized, at least at some 

 period of their development, by an internal asymmetry resulting 

 from a torsion of the body. In this the digestive canal, which in 

 the very young embryo (up to the trochosphere stage) has 

 mouth and anus at opposite ends of the body, becomes flexed 

 upon itself and twisted upw^ard, bringing the anus to the anterior 

 end, above the mouth. This torsion is connected with the in- 

 creasingly great development of the foot and the enlargement 

 of the main visceral mass, the liver and reproductive organs. 

 In the train of this torsion follow various displacements of the 



