836 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 perseon 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 setse 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 perseon 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 farther back 

 than in the male ; the mandibles are said to be wholly 

 lost ; the maxillipeds are reduced. The first gnathopods 

 wdth 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 peragon and the pleon 

 are as in the male, but the fourth, fifth, and often the 

 sixth segments of the per^eon are fused, the dilated trans- 

 parent skin affording a view of the young ones within. 



