MOLLUSCA. 239 



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 develop- 

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



276. A very valuable laboratory exercise may be had by comparing 

 large numbers of shells of a single species, found under varying condi- 

 tions. Compare as to shape, markings, etc., and see whether there are 

 individuals connecting your extreme groups. The land snail {Helix, Fig. 

 119) is favorable for such study. 



277. 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? 



What are your conclusions as to the symmetry and the normal position 

 of the squid ? Do you find anything from your external examination which 

 would lead you to class it with the clam and the snail? 



DESCRIPTIVE TEXT. 



278. The group Mollusca embraces from 10,000 to 20,- 

 ooo living species among which there are very great differ- 

 ences, as illustrated by forms as unlike as slugs, snails, oysters, 

 clams, devil-fishes, and squids. With the exception of a few 

 they are sluggish animals, and largely aquatic or frequenters 

 of moist places. Some are well protected by external armor 

 and others are perfectly naked. The typical adult mollusk 

 is clearly marked off from both the radiate animals such as 

 echinoderms and the segmented animals such as the Arthro- 

 pods and the Annulata, but some of the simpler types of 

 mollusks, and the larvae of certain of them which undergo a 

 metamorphosis, strongly suggest that they may be related to 

 some of the unsegmented worms. 



279. General Characters. 



i. Body soft, unsegmented, bilaterally symmetrical and 

 without segmented appendages. 



