CRUSTACEA. 491 



this rudimentary Nauplius stage into a stage corresponding with the Cyclops 

 stage of the free forms (fig. 230 B). In the Cyclops stage the larva has an 

 elongated body with a large cephalo-thoracic shield, and four free posterior 

 segments, the last of which bears a forked tail. 



There are now present eight pairs of appendages, viz. antennas (two 

 pairs), mandibles, maxillae, maxillipeds, and three pairs of swimming feet. 

 The Nauplius appendages are greatly modified. The first pair of antennas is 

 three-jointed, and the second biramous. The outer ramus is the longest, and 

 bears a claw-like bristle at its extremity. This pair of appendages is used 

 by the larva for fixing itself. The mandibles are small and connected with 

 the proboscidiform mouth ; and the single pair of maxillae is small and palped. 

 The maxillipeds (/;' and/;;/ 2 ) are believed by Claus to be primitively 

 a single biramous appendage, but early appear as two distinct structures 1 , 

 the outer and larger of which becomes the main organ by which the larva is 

 fixed. Both are at this stage simple two-jointed appendages. The two 

 anterior pairs of swimming feet have the typical structure, and consist of a 

 protopodite bearing an unjointed exopodite and endopodite. The first pair 

 is attached to the cephalo-thorax and the second (/ 2 ) to the first free thoracic 

 segment. The third pair is very small and attached to the second free 

 segment. The mouth is situated at the end of a kind of proboscis formed 

 by prolongations of the upper and lower lips. The alimentary tract is fairly 

 simple, and the anus opens between the caudal forks. 



Between this and the next known stage it is quite possible that one 

 or more may intervene. However this may be the larva in the next stage 

 observed (fig. 230 C) has already become parasitic in the mouth of the Perch, 

 and has acquired an elongated vermiform aspect. The body is divided into 

 two sections, an anterior unsegmented, and a posterior formed of five 

 segments, of which the foremost is the first thoracic segment which in the 

 earlier stage was fused with the cephalo-thorax. The tail bears a rudimen- 

 tary fork between the prongs of which the anus opens. The swimming feet 

 have disappeared, so also has the eye and the spiral duct of the embryonic 

 frontal organ. The outer of the two divisions of the maxilliped have under- 

 gone the most important modification, in that they have become united at 

 their ends, where they form an organ from which an elongated rod (/) 

 projects, and attaches the larva to the mouth or gills of its host. The 

 antennae and jaws have nearly acquired their adult form. The nervous 

 system consists of supra- and infra-oesophageal ganglia and two lateral 

 trunks given off from the latter. At this stage the males and females can 

 already be distinguished, not only by certain differences in the rudimen- 

 tary generative organs, but also by the fact that the outer branch of the 

 maxillipeds is much longer in the female than in the male, and projects 

 beyond the head. 



In the next ecdysis the adult condition is reached. The outer maxilli- 



1 Van Beneden (No. 506) in the genera investigated by him finds that the two 

 maxillipeds are really distinct pairs of appendages. 



