MOLLUSCOUS ANIMALS. 325 



Tunicata ; Sth, Polyzoa often spoken of by a name given soon after 

 the original one, Bryozoa. 



Before I examine more particularly the pretensions of these sup- 

 posed classes, it may be useful to explain the meaning and use of sub- 

 classes in the arrangement of the animal kingdom. It is as a matter 

 of fact, not unfrequently found, that where the structure corresponds 

 in the main, and essentially conforms to the same type, there may be 

 distinguished two or more grades of development, in each of which ana- 

 logous secondary groups occur, so that they resemble classes diifering. 

 more in degree than in kind, and bound together by a strong and well- 

 marked resemblance, though each having its own secondary divisions. 

 In such cases, the numerous objects all formed on a common plan, are 

 accounted as one class, whilst the different grades of development 

 mark sub-classes. In illustration we may refer to Owen's view of the 

 classification of Mammalia where the great divisions founded on the 

 character of the brain, which, though strongly objected to by some^. 

 are probably good, may be accounted as sub-classes, and under eacli 

 of them great families corresponding in number and analogous i^: 

 structure and habits may be pointed out. Again in the ClassificatioB 

 of Birds, the great division of Perching birds is clearly one sub-class, 

 whilst the remainder of the received orders form another, and the 

 secondary divisions of the perchers correspond with the other orders. 

 The same might be shown in the case of Fishes, and there are other 

 well established examples. Now no one can well doubt that the class 

 Cephalopoda exhibits the highest development and the greatest per- 

 fection of the organs of sense among the Mollusca. It is scarcely less 

 certain that the Pteropoda excell all other Molldsca in activity and 

 muscular power, and form, though a small, a very distinct class corres- 

 ponding in position with the Articulata among the Sub-kingdoms. 

 Cuvier's Heteropoda being disposed of in the manner already indi- 

 cated, we come to Gasteropoda, the special nutritive type, correspond- 

 ing with Mollusca among the sub-kingdoms and thus as being pecu- 

 liarly typical, the most numerous and varied of all the classes. 



It seems highly probable that all the remaining Mollusca, which 

 are without a distinct head, constitute two classes, according as they 

 have the mantle split open, its portions being covered by a pair of 

 shells meeting in a hinge ; or forming an undivided sac, with incurrent 

 and excurrent openings, and its surface either of leathery or horny 

 consistency. In each case we have two sub-classes, distinguished by 



