CLASSIFICATION 



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 form- 

 ing 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 Gas- 

 teropoda, with special adaptations to pelagic life, and are there- 

 fore 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,^ 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)^'^ 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 risa 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. 



1 7Xw(ro-o, tongue ; <f>^peiv, to carry. 2 \eiireiv, to be wanting. 



