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 MOLLUSCA | +----------------+----------------------------------+ | | =Glossophora= =Aglossa= | | +--------+----------+-------------------+ | | | | | _Cephalopoda_ _Gasteropoda_ _Scaphopoda_ _Pelecypoda_ =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_.