I2 4 



THE CRUSTACEA 



several setae a large olfactory filament or " aesthetasc." The 

 antennae are reduced to shapeless vestiges and soon disappear 

 entirely. The labrum becomes greatly reduced in size. The 

 mandibles are represented only by the gnathobasic portion, the rest 

 of the limb being reduced to a papilliform "palp." The maxillulae 

 and maxillae are closely crowded together in nearly the position 

 which they occupy in the adult, and form, with the mandibles and 

 labrum, a prominent buccal mass. At this stage all the mouth- 

 parts are devoid of setae and are not functional. The six pairs of 

 thoracic limbs are now well developed. Each consists of a proto- 

 podite of two segments and two rami, each also bi-segmented, 

 carrying long natatory setae. The " thoracic " region of the body 

 is indistinctly segmented, and is followed by a small limbless 



rf 



eye ; 1, antennule, with adhesive sucker. 



(After Claus, from Korschelt and Heider's 



abdomen of four segments terminating in a short setose caudal 

 fork. 



As regards the internal anatomy, the unpaired eye (ua) persists 

 and is accompanied by a pair of large compound eyes (pa) which 

 were already visible in the last metanauplius stage. Paired 

 diverticula from the anterior portion of the alimentary canal form the 

 rudiments of the digestive gland, and a rudiment of the ovary is also 

 visible in the anterior region. Finally, the Cypris larva becomes 

 permanently attached by means of the antennules, and considerable 

 changes go on under the cuticle in preparation for the next moult. 



The most important of these changes (Fig. 75, A, B) are the 

 development of the long cirriform thoracic limbs of the adult, and 

 a concomitant change in the position of the thoracic region of the 

 body which comes to lie at right angles to the long axis of the 



