CRUSTACEA 



377 



a mantle which encloses the trunk; with usually a mandibular palp, 

 which is never biramous; and typically with six pairs of biramous 

 thoracic limbs. 



The great majority of the Cirripedia are extremely unlike the rest 

 of the subphyllum, and would not be recognized as crustaceans at all 

 by the layman. The familiar members of the class are the ordinary 

 barnacles (Thoracica). Besides these, however, it contains several 

 groups of related organisms, of which the parasitic barnacles (Rhizo- 

 cephala) are the best known. The Ascothoracica link the class to other 

 crustaceans. 



,an . 



sip.- — ^rrrrz^ 



Fig. 254. Arguliis. A, A ventral view of a female of A. americanus. From 

 Caiman, after Wilson. B, The second left swimming limb of A. foliaceus. 

 After Hansen. An. position of anus; an.' antennule; an." antenna; e. paired 

 eye; ex. exopodite; mx.' maxillule; mx." maxilla; ram. ramus of caudal furca; 

 in some species the rami stand immediately on each side of the anus; 

 sip. siphon, or suctorial proboscis; sp. poison spine. 



Order THORACICA 



Cirripedia with an alimentary canal ; six pairs of biramous thoracic 

 limbs; no abdominal somites; and permanent attachment by the 

 preoral region. 



We shall take as an example of this group the common goose 

 barnacle, Lepas (Figs. 255, 257 A), found all the world over on float- 

 ing objects in the sea. It hangs by a stalk or peduncle which, as we 

 shall see, represents the foremost part of the head, greatly elongated 



