10 CIRRIPEDIA, 



phoses, since which time, Cirripedes have been almost uni- 

 versally admitted amongst the Crustaceans. It is well 

 known, that it is hardly possible to give a definition of this 

 great class, which shall include every member of it ; never- 

 theless, even if the mature Cirripede alone be considered, 

 the following characters, viz. the slight separation of the 

 head and thorax, the latter generally bearing six pairs of 

 appendages, and the being enclosed in a carapace — together 

 with the periodical exuviation of the greater part of the 

 external membranes, would, perhaps, suffice to show that it 

 should be classed amongst Crustacea. 



But it still remains undecided what rank in this class 

 Cirripedes should hold. Before briefly discussing this point, 

 it is indispensable to indicate their essential characters, 

 which 1 will immediately attempt. For as long as it remained 

 doubtful which was their anterior extremity, which the ven- 

 tral or dorsal surface ; as long as the peduncle was thought 

 by one naturalist to be the legs, by another the abdomen, 

 in a modified condition, it was hopeless to compare Cirripedes 

 with ordinary Crustaceans, and assign to them their due 

 rank. 



In the larva in the first stage, an eye and two pairs of 

 antennas are in process of formation or are developed ; here, 

 then, according to the analogy of all Crustaceans, we have 

 evidence of the existence of the first three cephalic segments. 

 The mouth always consists of three pairs of gnathites, 

 and hence again, from analogy, this part may be inferred 

 to be formed of, and supported on, three other segments ; 

 making thus far six segments. In two Orders out of the 

 three into which Cirripedes may be divided, namely, in the 

 Abdominalia and Apoda, eight quite distinct segments suc- 

 ceed the mouth ; of these the first differs slightly from the 

 seven succeeding segments, and may, I think, be safely 

 considered as forming the seventh (cephalic) segment. 

 The next seven segments resemble each other in all 

 essential respects, and are no doubt the normal, seven tho- 

 racic segments. These, in both the above orders, are suc- 

 ceeded by three smaller segments, which differ in structure 

 from the thoracic segments, and must be abdominal. Hence 

 we here have, altogether, seventeen segments. It should, 



