342 



MARINE INVERTEBRATES 



Having examined the animal which inhabits the shell and 

 gained an idea of its anatomy, we may now turn to an inspection 

 of the house which it has built for itself, and here we shall find a 

 delightful subject for studj r . 



THE GASTEROPOD SHELL 



An industrious hour of collecting upon almost any beach will 

 provide the student with an abundance of interesting shells. 



Putting aside the bivalves, the uni- 

 valve or gasteropod shells remain- 

 ing should be closely examined for 

 the purpose of becoming acquainted 

 with their various conchological 

 features. The collector will see 

 at once that he has a number of 

 species that differ more or less 

 widely in their general form, size, 

 texture, and sculpture. Some 

 shells will be long in proportion to 

 their width, with many turns of the 

 spire which probably terminates 

 in a sharply pointed apex ; others 

 will be almost round, with com- 

 paratively few spiral turns which 

 end in a blunt apex, giving to the 

 specimen outlines suggestive of a 

 dome. A wide range of characters will be found in the mouths 

 or apertures of the shells, some being almost if not quite round, 

 others oblong and with a notch cut into the lower portion of the 

 opening, and others possibly with this notch extended into a sort 

 of semi-inclosed channel. A first lesson in classification of the 

 Mollusca may be taken by dividing the results of the first day's 

 collecting of univalve shells into groups according to these promi- 

 nent shell-characters. 



A good knowledge of the parts of a gasteropod shell is essential 

 to the student, and it can readily be acquired. Four specimens are 

 given in the accompanying figures, representing highly divergent 



Parts of a gasteropod shell : a, anterior 

 canal ; b, body-whorl ; c, colnmella ; I, outer 

 lip ; m, mouth or aperture ; p, posterior 

 canal; , sutures; p, spire; n, umbilicus. 



