and the Elements in which they live. 165 



lusca, and unite the Arthropoda, Crustacea and Insects to form 

 another group of equal value. 



The great diversity among worms seems at first to warrant, 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 notwithstanding the many pecu- 

 liarities which characterize 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. As- 

 suming upon the foundation alluded to, and without entering 

 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 subdivided. 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 combination of the difl'erent 

 groups of Articulata will bring them all into three classes, one 

 containing those in which the body is either more or less di- 

 stinctly articulated, or in which indications of transverse wrinkles 

 in the skin are scarcely marked or wholly wanting, but in which, 

 however developed 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 locomo- 

 tive appendages may be, and in which the foremost joints hardly 

 ever assume a peculiar structure with the appearance of a head. 

 This class, for which the name of Worms is best retained, will 

 contain the Helminths and Annelides, exclusive however of the 

 vermiform parasitic Crustacea, which embryology has taught us 

 to refer unhesitatingly to the class of Crustacea. The extraor- 

 dinary diversity which exists among these animals renders it 

 rather difficult to subdivide them into natural groups, and to as- 

 sign to these groups a natural succession agreeing with the gra- 

 dation of their structure, as there are so many, the development 

 of which is as yet very imperfectly known, and others which un- 

 dergo so complicated metamorphoses as to leave great doubt re- 

 specting their natural relations to each other. However, there 

 can be no doubt that the Helminths rank lower than the Anne- 

 lides, for their structure indicates plainly their inferiority, and 



