1 62 INTRODUCTION TO ZOOLOGY. 



school, with better directed zeal and a higher aim, and 

 numbers became disciples when they saw that here as much 

 satisfaction and profit were to be reaped as in the study of 

 almost any other class ; for it may be laid down as an axiom, 

 that no branch of natural history, however apparently trifling, 

 "but may be ennobled by the manner in Avhich it is pursued; 

 and when the student carries all its Avonders back to the one 

 Great Source, the smallest worm, and the most beautiful of 

 his own species, will afibrd him subjects for the deepest 

 contemplation." 



We now proceed to examine some of the leading divisions 

 of the mollusca. The first and most obvious is into two gi'eat 

 groups, one containing those which, like the common oyster, 

 are destitute of a head (Acephala); and the other those 

 which, like the snail, are provided with a head, and generally 

 with mouth, eyes, and tentacula (Encejjhala).* Each group 

 is divided into three classes — the former " according to the 

 modifications of the integument or of the gills ; " the latter, 

 according to those of the locomotive organs. We shall briefly 

 notice the characteristics of these six classes, and enumerate 

 some of the best known examples of each. 



* The names of the classes into which the mollusca are divided may 

 be exhibited thus: 



ACEPHALA. 



I. Tunicata with a cloak or tunic 



II. Brachiopoda arra.-footed. 



III. Lamellibranchiata plate-shaped gills. 



ENCEPIIALA. 



IV. Pteropoda wng-footed. 



V Gasteropoda belly-footed. 



VI. Cephalopoda head footed. 



