218 A HISTORY OF EECENT CKUSTACEA 



whereas the secondary is dilated and longitudinally hollowed 

 so that its companion can be sheltered within it when not 

 in use, but at other times the two pairs of flagella together 

 form the efferent branchial tube, which is continued back- 

 wards by the peduncles of the first and the scales of the 

 second 'antennas, these making a broad channel between 

 the bases of the peduncles of the second antennse, where 

 it is closed in below by the mandibular ' palp,' and diverges 

 on each side of the upper lip into the passages from the 

 branchial chambers. The generic and specific names alike 

 signify ' a creature with channel- or pipe-forming antennae.' 



PleotlcuSj Spence Bate, 1888, also has the flagella of 

 the first antennas longer than the carapace, but w^ithout 

 the grooved arrangement. Its second antennae claim 

 notice as having the flagellum ' three times the length of 

 the animal, or more.' 



Sicyonia, Milne-Edwards, 1830, has its species, two of 

 which occur in the Mediterranean, distinguished for the 

 rigidity of the integument. The flagella of the first 

 antennge are very short ; there are no exopods to the 

 trunk-legs as there are in Penceus, and the pleopods are 

 all sing-le-branched. From Penceus it differs in the struc- 

 ture and arrangement of the branchiae, though agreeing 

 with it apparently in the absence of podobranchia?. In 

 defining the genus Spence Bate says that the second 

 maxillipeds carry ' a mastigobranchial plate without a 

 podobranchia,' ' one arthrobranchial and one pleurobran- 

 chial plume.' On the next page, after giving a scheme of 

 the branchiae of Sicyonia which includes six pleurobranchi^e 

 and no podobranchia, he states that it differs from Penceus 

 *in the absence of any traces of pleurobranchi^e, in the 

 reduction of the arthrobranchial plumes, and in the pre- 

 sence of one podobranchial plume attached to the first 

 pair of gnathopoda' [i.e. second maxillipeds]. Presently 

 after, in the description of Sicyonia carinata (Olivier), he 

 says of these same second maxillipeds that the first joint 

 * carries a long and slender mastigobranchia shaped like 

 that in Penoius, and, as in that genus, there is no bran- 

 chial plume attached to it.' Thus there both is and is not 



