208 L. Agassi z on the Natural Belations between 



question, whether Articulata in the widest extension of this 

 group constitute one single natural type, or whether they 

 should be subdivided into two equivalent groups, as has been 

 proposed by those who would restore the division of worms, 

 in its widest sense, as a great division equal in zoological 

 importance to the type of Mollusca, and unite the Arthro- 

 poda, Crustacea, and insects, to form another group of equal 

 value. 



The great diversity among worms, seems at first to war- 

 rant in some degree such an arrangement. But as soon as 

 we consider the metamorphosis which insects undergo, and 

 compare their earliest stages of growth with the structure 

 and forms of worms, we cannot fail to perceive, that notwith- 

 standing the many peculiarities which characterise worms, 

 they are, after all, only one of the permanent modifications 

 of the same type as Crustacea and insects, among which last 

 the characters and forms of a large number of worms are 

 reproduced as transient states of growth ; so that upon the 

 most natural view, and especially if we allow embryology to 

 have its due weight in fixing our opinion, we must consider 

 worms with all their diversified forms, Crustacea in all their 

 diversity, and Lepades, Arachnidse, and Insects, to constitute 

 one single undivided natural type in the animal kingdom. 

 Assuming upon the foundation alluded to, and without enter- 

 ing into a detailed argument upon this question, that this is 

 the right view of this subject, the next question is about the 

 number of classes into which these Articulata should be sub- 

 divided. Taking here again anatomical and embryological 

 evidence as our guide, and remembering what was said above 

 of intestinal worms, we shall find that the most natural com- 

 bination of the difi'erent groups of Articulata will bring them 

 all into three classes, one containing those in which the body 

 is either more or less distinctly articulated, or in which in- 

 dications of transverse wrinkles in the skin are scarcely 

 marked, or wholly wanting, but in which, however de- 

 veloped these joints may be, they never combine in such a 

 manner as to divide the body into distinct ridges, in which 

 the form is always elongated and vermiform, never provided 

 with articulated rings, however numerous and diversified the 



