OF THE ANNULOSA. 391 



One of the first properties of this compend of affinities 

 which will excite attention is, that the Annulosa appear 

 therein to be naturally divided into two great groups ; one 

 composed of three circles distinguished by a tendency to 

 metamorphosis and a bifurcated tracheal system of re- 

 spiration, the other of two circles characterized by the 

 possession of internal antennas and a tendency to a system 

 of circulation. The only animals indeed of these last 

 groups, which are subject to any remarkable change in 

 the primitive and essential form of their body, are a few 

 Branchiopod Crustacea; that is, the opposite points of 

 the circle of Annulosa correspond intimately with each 

 other. It is not a little singular that this correspondence, 

 or perhaps more properly this affinity, which exists be- 

 tween the Brajtchiopoda and the Hexapod Insects should 

 be founded on the nature of their metamorphosis. Some 

 credit is due to Miiller if he ever had this connexion in 

 view, and it is probable that he really had, from his having 

 bestowed on these Crustacea the appropriate appellation 

 of Entomostraca or Testaceous Insects. Even if he meant 

 no more by the word than that they are articulated testa- 

 ceous animals, it is well known that their disposition to 

 metamorphosis, so contrary to the usual habit of the Crus- 

 tacea, did not escape him. 



I have already made an attempt to draw the attention 

 of naturalists to the relations of analogy existing between 

 corresponding points of the two contiguous circles which 

 pass through a perfect change of form ; and now I shall 

 content myself with indicating, by position, those analo- 

 gies which apparently hold good between the correspond- 

 ing points of all the five groups of Annulosa. While, 

 however, at this part of my subject, I must express regret 

 at knowing comparatively so little of the Crustacea and 



