-178 HISTORY OF PHYSIOLOGY. 



that the skull of all vertebrate animals is pretty well reduced to a uni- 

 form structure, and the laws of its variations nearly determined. 7 



The vertebrate animals being thus reduced to a single type, the 

 question arises how far this can be done with regard to other animals, 

 and how many such types there are. And here we co:aae to one of 

 the important services which Cuvier rendered to natural history. 



Sect. 2. Distinction of the General Types of the Forms of Animals. 



Cuvier. 



ANIMALS were divided by Lamarck into vertebrate and invertebrate; 

 and the general analogies of all vertebrate animals are easily made 

 manifest. But with regard to other animals, the point is far from 

 clear. Cuvier was the first to give a really philosophical view of the 

 animal world in reference to the plan on which each animal is con- 

 structed. There are, 8 he says, four such plans ; four forms on which 

 animals appear to have been modelled ; and of which the ulterior di- 

 visions, with whatever titles naturalists have decorated them, are only 

 very slight modifications, founded on the development or addition of 

 some parts which do not produce any essential change in the plan. 



These four great branches of the animal world are the vertebrata, 

 mollusca, articulata, radiata ; and the differences of these are so im- 

 portant that a slight explanation of them may be permitted. 



The vertebrata are those animals which (as man and other sucklers^ 

 birds, fishes, lizards, frogs, serpents) have a backbone and a skull 

 vvith lateral appendages, within which the viscera are included, and to 

 which the muscles are attached. 



The mollusca, or soft animals, have no bony skeleton ; the muscles 

 are attached to the skin, which often includes stony plates called 

 shells ; such molluscs are shell-fish ; others are cuttle-fish, and many 

 pulpy sea-animals. 



The articulata consist of Crustacea (lobsters, &c.), insects, spiders, 

 and annulose worms, which consist of a head and a number of succes- 

 sive annular portions of the body jointed together (to the interior of 

 which the muscles are attached), whence the name. 



Finally, the radiata include the animals known under the name of 

 zoophytes. In the preceding three branches the organs of motion and 

 of sense were distributed symmetrically on the two sides of an axis, 



1 Cuv. Hist. Sc. Nat. iil. 442. e Rigne Animal, p. 57. 



