218 A HISTORY OF RECENT CRUSTACEA 
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 antennz, these making a broad channel between 
the bases of the peduncles of the second antennz, 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 antenne.’ 
Pleoticus, Spence Bate, 1888, also has the flagella of 
the first antenne longer than the carapace, but without 
the grooved arrangement. Its second antennz claim 
notice as having the flagellum ‘three times the length of 
the animal, or more.’ 
Sieyonia, 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 
antennz are very short; there are no exopods to the 
trunk-legs as there are in Penceus, and the pleopods are 
all single-branched. From Penceus it differs in the struc- 
ture and arrangement of the branchiz, though agreeing 
with it apparently in the absence of podobranchiz. 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 branchiz of Sicyonia which includes six pleurobranchize 
and no podobranchie, he states that it differs from Penceus 
‘in the absence of any traces of pleurobranchie, in the 
reduction of the arthrobranchial plumes, and in the pre- 
sence of one podobranchial plume attached to the first 
pair of gnathopoda’ [7.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 Penceus, and, as in that genus, there is no bran- 
chial plume attached to it.’ Thus there both is and is not 
