PROGRESS OF ANIMAL MORPHOLOGY. 493 



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 norms, which 

 consist of a head and a number of successive 

 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, so that the animal has a right and a left 

 side. In the radiata the similar members radiate 

 from the axis in a circular manner, like the petals 

 of a regular flower. 



The whole value of such a classification cannot 

 be understood without explaining its use in enabling 

 us to give general descriptions, and general laws 

 of the animal functions of the classes which it in- 

 cludes; but in the present part of our w r ork our 

 business is to exhibit it as an exemplification of the 

 reduction of animals to laws of symmetry. The 

 bipartite symmetry of the form of vertebrate and 

 articulate animals is obvious ; and the reduction of 

 the various forms of such animals to a common 

 type has been effected, by attention to their ana- 

 tomy, in a manner which has satisfied those who 

 have best studied the subject. The molluscs, espe- 

 cially those in which the head disappears, as oysters, 



