A, Ommastrephes sagittatus Lam., Naples: a, a, arms surrounding the mouth; f, funnel; t, t, the two ‘tentacular’ arms, × ⅖. B, Buccinum undatum L., Britain: f, foot; pr, proboscis. × ½. C, Dentalium entalis L., Norway: f, foot. D, Cardium oblongum Chem., Naples: f, foot; s, efferent or anal siphon; s’, efferent or branchial siphon, × ½.
The Pelecypoda[5] are enclosed in a bivalve shell fastened by a muscular hinge, the adjacent part of the valves being generally more or less toothed; the foot is as a rule roughly comparable to the shape of an axe-head.
To these four Orders is frequently added a fifth, the Pteropoda, whose exact position is at present not absolutely settled. The Pteropoda[6] are ‘pelagic,’ i.e. they live in the open waters of the ocean, rising to the surface at night, and sinking into cooler water by day. They are provided with a pair of wing-like appendages or ‘feet,’ on each side of the head, by means of which they are enabled to swim. Some authorities regard the Pteropoda as a subdivision of Gasteropoda, others as forming a separate Order, of equivalent value to the other four. The question will be further discussed below (see chap. [xv].), but for the present it will be sufficient to state that the weight of evidence appears to show that the Pteropoda are modified Gasteropoda, with special adaptations to pelagic life, and are therefore not entitled to rank as a separate Order.
Some writers conveniently group together the first three of these Orders, the Cephalopoda, Gasteropoda, and Scaphopoda, under the title Glossophora,[7] or Mollusca furnished with a radula or ribbon-shaped ‘tongue,’ set with rows of teeth and situated in something of the nature of a head, as distinguished from the Aglossa (or Lipocephala),[8] i.e. those Mollusca which have no radula and no head. To the latter belong only the fourth Order, the Pelecypoda. This view postulates, for the primitive ancestral Mollusc, a body with a more or less developed head, and possibly the rudiments of an apparatus for grinding or triturating food. This form, it is held, either developed or degenerated. In the former case, in consequence of the more active mode of life upon which it may be supposed to have entered, it gave rise to all the more highly organised forms which are grouped under the three great Orders. When, on the other hand, the ancestral form associated itself with an inactive or sedentary life, it was, we may believe, modified accordingly, and either lost by atrophy or failed to acquire those special points of organisation which characterise the highly-developed form. Hence the Pelecypoda, or bivalves, whose characteristic is the absence of any definite cephalic region or masticatory apparatus. It is a remarkable fact in support of this theory of the origin of the Aglossa that certain of their larvae are known to possess traces of higher organisation, e.g. an external mouth and eyes, the former of which becomes covered by the mantle lobes, while the latter disappear long before the adult stage is reached.
Thus we have
Classification of Gasteropoda.—The Gasteropoda are numerically very largely in excess of the two other Orders of the Glossophora, far more complicated as regards classification, and contain a large proportion of those examples of the Mollusca which are most familiar to the ordinary observer. It will therefore be convenient to postpone for the present a fuller discussion of the subdivisions of the Cephalopoda and Scaphopoda, as well as of the Aglossa, returning to them again in special chapters (chaps. xiii. and xvi.), and to devote a few introductory words to the classification and relations of the Gasteropoda.
The Gasteropoda are divided into four Classes, Amphineura, Prosobranchiata, Opisthobranchiata, and Pulmonata.
Fig. 2.—An example of the Polyplacophora: Chiton spinosus Brug.