There is nothing remarkable about the vascular system to distinguish it from that of many higher forms of animals. It is, however, not completely closed—by which is meant that the blood is not always contained within arteries or veins, and that it does sometimes flow into other organs and floods certain other body-cavities, although the vascular system of mollusks is by no means so completely open as is that of insects and crustaceans. [pg342] 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 study. 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 univalve or gasteropod shells remaining 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 comparatively 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 prominent shell-characters. [Illustration: Parts of a gasteropod shell: /a/, anterior canal; /b/, body-whorl; /c/, columella; /l/, outer lip; /m/, mouth or aperture; /p/, posterior canal; /s/, sutures; /sp/, spire; /u/, umbilicus.] 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 [pg343] types of marine gasteropods, and their respective characteristics may be noted by comparing them one with another: /Fasciolaria tulipa/ (Plate LXXVI.), a Floridian shell; /Polynices heros/, a common species found on all the sandy shores of the American Atlantic coast; /Murex tenuispina/, an Indo-Pacific species much prized by collectors; and /Acmæa testudinalis/, commonly found in Maine and Massachusetts at low tide clinging to rocks. [Illustration: /Polynices heros/; /Acmæa testudinalis/, from below; /Acmæa testudinalis/, profile; /Murex tenuispina/.]