OF THE ANNULOSA. 355 



the other two segments. These last seem, on a first glance, 

 to belong to the abdomen rather than to the thorax, and 

 indeed form one portion of the body with the former; 

 their lower side or pectus supporting the other two pairs 

 of feet. Now, among the parasitical Anoplura we find 

 the same disposition of the annular segments of the body, 

 and some of the Ricini of Degeer have the first segment 

 of the trunk dilated into a thorax in the same manner. 



It would be improper to proceed to the Winged insects, 

 or those which undergo a true metamorphosis, without 

 recalling to the reader's recollection the substance of a few 

 remarks which were made in the preceding chapter on the 

 Ametabola. The Annelides, it will not be forgotten, are 

 destitute of a heart, although their circulation is distinct- 

 ly effected by the aid of dorsal and ventral vessels. Now 

 the Ametabola have the dorsal canal of the Annelides, 

 but no distinct circulation; and the Crustacea which fol- 

 low are furnished with a heart and system of circulation 

 more analogous to those of fishes than exist either in the 

 Annelides or Ametabola. Hence it is plainly to be inferred, 

 that, setting external form and consistence aside, the cir-, 

 culation of itself, were it allowable to argue on it alone, 

 is sufficient to allay every doubt on the point that the 

 Ametabola come nearer to the Annelides than the Crus- 

 tacea. On the other hand, there is little further simila- 

 rity between the Crustacea and the Vertebrata than that 

 which depends on the system of circulation ; and if we 

 found our arrangement on such a principle alone, we can- 

 not expect otherwise than to fall into that confusion which 

 is the- invariable consequence of reasoning on single or 

 instated phenomena. Still the system of circulation in 

 Crustacea is very interesting, and such as ought to give 

 2 A 2 



