THEIR RANK. ] 1 



however, be observed that in the two orders just referred 

 to, each includes only a single species ; but I know of no 

 good reason why, on this account, their structure should be 

 valued the less. In the third order, the Thoracica, which 

 includes all common Cirripedes, two segments with their 

 appendages are missing out of the eight that should succeed 

 the mouth ; from the open interval in the pupa, between 

 the mouth and first pair of natatory legs, and from some 

 other reasons, I believe that the two missing segments are 

 the seventh and eighth, or last cephalic and first thoracic 

 segments, and that they have coalesced close posteriorly to 

 the mouth.* In the order Thoracica, the abdomen is quite 

 rudimentary, though often still bearing caudal appendages ; 

 in the pupa, however, of this order, as in the mature animal 

 of the two other orders, it is formed of three segments. 

 Hence I conclude that, notwithstanding the absence of the 

 above two segments with their appendages in the Thoracica, 

 the archetype Cirripede may be safely said to be composed 

 of seventeen segments. 



In the classification of Crustacea, the relation and number 

 of the segments of the different parts of the body, are viewed 

 both by Prof. Milne Edwardsf and Mr. Dana,j as of the 

 highest importance. I may premise that both these au- 

 thors divide the Crustacea into Podophthalmia, Edriophthal- 

 mia, and Entomostraca ; Milne Edwards making a fourth 

 legion, the Branchiopoda, and another division, including 

 Limulus, of equal value to the above four legions altogether; 

 whereas Dana sinks Limulus and the Branchiopoda under 

 his Entomostraca. As far as concerns our present discussion 

 on Cirripedes, the first three divisions, as valued by Dana, 

 will best serve as standards of.comparison ; but it is not 

 unimportant to our present purpose, as showing how diffi- 

 cult it is to weigh the value of the higher divisions of a 

 Class, to observe the wide difference in opinion of two natu- 

 ralists, so eminent for their knowlege of the class in question 

 and for their high abilities. 



* This question and the whole subject of the homologies of the several parts 

 of a Cirripede, will be discussed under the head of the Metamorphoses of the 

 Balanidse. 



f Annales des Sciences Nat., torn, xviii, p. 120, 1852. 



X Crustacea: 'United States Exploring Expedition,' p. 1395, 1852. 



