336 A HISTORY OF RECENT CRUSTACEA 
The first five segments of the pleon are distinct, carrying 
each a pair of pleopods; the sixth segment ends in a 
pointed telson and bears inserted at the sides of the base 
a pair of two-branched uropods resembling the pleopods 
except in being of a firmer texture. 
In the adult male the so-called mandibles are powerful 
and exserted beyond the front of the quadrate head; the 
maxillipeds have a ‘ palp’ consisting of four flattened 
ciliated joints. The first segment of the pereeon is sepa- 
rated from the head only by a suture, and its appendages, 
the first gnathopods, are (in Gnathia) two-jointed, oper- 
cular, the first joint being a large pyriform plate fringed 
with setz on the convex. inner margin and containing 
three semitransparent calcareous plates, supposed to in- 
dicate the same number of original joints. The seventh 
segment of the perzeon is abruptly narrower than the pre- 
ceding, so that it seems to form part of the narrow pleon. 
The pleopods with their two one-jointed rami are sometimes 
ciliated, and sometimes not. Dr. Dohrn considers that 
the so-called mandibles of the adult male are structures 
that arise independently of the true mandibles as found in 
the young. He considers that these new structures are 
not concerned in feeding, but only in attaching the animal 
to some object. For feeding purposes he states that the 
opercular gnathopods are thrown open, and the maxilli- 
peds act as whirling organs, the current of water so 
maintained bringing with it small nutritious particles such 
as do not require any powerful oral apparatus for their 
mastication. 
In the adult female the head is subtriangular, with 
the eyes (when present) larger and placed further back 
than in the male; the mandibles are said to be wholly 
lost; the maxillipeds are reduced. The first gnathopods 
with the joints unexpanded or not much expanded lie each 
on a delicate membranous plate which may be marsupial. 
The first and last segments of the perzeon and the pleon 
are as in the male, but the fourth, fifth, and often the 
sixth segments of the perzeon are fused, the dilated trans- 
parent skin affording a view of the young ones within. 
