74 VERTEBRATED TYPE OF STRUCTURE. 



liver, chiefly on the right side ; the stomach and pancreas on the 

 left, and other parts in the same irregular manner. The 

 respiratory organs, however, always display more symmetry; 

 probably, for this reason, that they are more closely connected 

 than any other of the organs of nutrition, with the apparatus of 

 muscular motion, by which their actions are kept up. (ANIMAL 

 PHYSIOL. Chap. VI.). But it is interesting to observe that, in 

 Birds the class of Vertebrata most distinguished for activity of 

 motion there is a more complete symmetry than is found among 

 Mammals, thus affording another exemplification of the general 

 rule just stated. The peculiarity in the construction of Verte- 

 brated animals, however, has reference not so much to their 

 external conformation, as to their possession of an internal 

 skeleton ; expressly adapted to give support and protection to 

 the nervous system, which in this class attains its highest degree 

 of development, and to afford fixed points of attachment to the 

 muscles. In this respect, therefore, they are readily distinguishable 

 from the Articulata, whose skeleton is external ; and from the 

 Mollusca, most of which have no skeleton at all. But as among 

 the highest of this last group, there are some (of the Cuttle-fish 

 tribe) which have the rudiments of an internal skeleton so 

 there are some among Fishes, in which the internal skeleton 

 characteristic of the Vertebrated series is so little developed, 

 that it has been doubted whether these animals deserve to be 

 ranked in it at all. So that, well marked and decided as the 

 line would seem to be, which separates the Vertebrata from 

 Invertebrated animals, we may be obliged to hesitate through 

 what exact points to draw it. 



52. In the preceding sketch of the four great plans, on which 

 the ever- varying forms of the Animal Kingdom appear to have 

 been constructed, it has not been attempted to embody all the 

 characters, by which they are respectively distinguished ; but to 

 give to the young Naturalist such an idea of each, as may aid hii 

 in that first act of classification, which consists in ascertaining 

 to which type a new or doubtful animal is to be referred, 

 more detailed account of each will be given at the comment 

 ment of that division of the work, which treats of the classes ii 



