236 



CRUSTACEA. 



Among the transformations which take place in the internal organs during 

 the Cxjdops stages, we must mention the degeneration of the paired optic 

 rudiment and of the secondary brain. Now for the first time the nerves which 

 mn to the paired frontal organs are distinctly recognisable. The antennal 

 gland degenerates, while the shell-gland, which opens externally at the base 

 of the anterior maxillipede, becomes functional in its stead. The anal aperture 

 no longer lies dorsally, but shifts to the posterior end of the body between 

 the two furcal appendages. The genital organs show an advance by an increase 

 in number of genital cells and by the development of the efferent ducts. The 

 paired genital rudiments now meet above the intestine and fuse to form a 

 single gland. The heart develops between the first and second thoracic 

 segments out of a paired rudiment of mesoderm-cells. 



Fig. 112.— Two stages of development of Canthommptus staphylinus (after Claus). A, Meta- 

 nauplius stage. B, Cyclops stage with three pairs of swimming limbs, a', first antenna ; 

 a", second antenna; md, mandible ; mx, maxilla; mf, second maxilla (rudiment of the two 

 so-called maxillipedes) ; mfi, first so-called maxillipede ; tnfn, second so-called maxillipede ; 

 pi, pii, pill, first, second, and third jiairs of thoracic limbs. 



In the seco7id Cyclops stage (Fig. 112 B), there first appear tlie 

 rudiments of the limbs of the fourth thoracic segment and the 

 delimitation of the fifth thoracic segment. Tlius we find, at this 

 stage, behind the cephalo-thorax, four free thoracic segments, and, 

 following these, the as yet unsegmented abdomen. Of the thoracic 

 limbs, the three anterior pairs are well developed. In the third 

 Ci/dops stage, the fourth pair of thoracic limbs has attained full 

 development and tlie first abdominal segment has formed ; in the 

 following Cgclops stages, the gradual segmentation of the abdomen 

 takes place, as well as the complete transition of the limbs to their 



