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4 CLASSES OF THE MALACOZOA — ENCEPHALA. 



appreciable degree strengthened by geological research ; the 

 earliest fossil vertebrate, mollusk or radiate being as clearly 

 characteristic of its type as is its modern representative. It is 

 only in the subordinate groups that we know differential charac- 

 ters to have developed from a common ancestral form. 



The "Vertebrates are usually spoken of as the highest form of 

 animal existence, the Protozoa as the lowest ; but it must be 

 borne in mind that the one is as well fitted as the other to 

 subserve the purposes of its life, that each in its organization is 

 equally perfect. These terms, then, indicate complex and simple 

 rather than perfect and imperfect organization. 



CLASSES OP THE MALACOZOA, 



True mollusks are broadly divided into two great groups by 

 important external characteristics : 



1. Encephala. The animal possesses a head, and is usually pro- 

 tected by a spiral shell. Example, the snail, 



2. Acephala. The animal is without a head, and is always 

 protected by a shell consisting of two pieces applied to its sides, 

 and connected at the back by a ligament or hinge. Example, 

 the oyster. 



To these two divisions are also familiarly applied names 

 describing the nature of their shelly coverings ; the first being 

 known as univalves, the last as bivalves. These designations 

 are partially incorrect and insufficient, as we shall soon see ; but 

 as the collection, preservation and study of shells is made with 

 facility, whilst the animals or soft parts are more difficult 

 to obtain and preserve, the former have imposed upon us a 

 designation which may be well enough understood but is scarcely 

 correct ; it would be preferable to call our science Malacology, 

 the study of molluscous or soft animals, yet it is almost uni- 

 versally known as Conchology, the Science of Shells. 



I, ENCEPHALA, 



Herein are included more than three-fourths of the whole 

 number of molluscous animals including four out of the five 

 classes into which the mollusca proper are divided. These 

 classes take their names from and are primarily founded upon 

 their respective locomotive organs. 



Class I. Cephalopoda. Cuttle-fish, argonauts, and their allies, 

 so called from the circle of feet, or more properly arm-like limbs 

 enclosing the mouth and arising from the top of the head (PI. 

 xxiii). 



Class II, Ptei^opoda. Animals inhabiting the high seas, swim- 

 ming by means of a pair of wings, extending laterally from the 

 back of the head, Clio, Hyalsea, etc, (PI. xlii). 



