234 CRUSTACEA. 



The body, which is usually oval (only in individual cases long, trans- 

 versely broadened or barrel-shaped), shows no traces of external 

 segmentation, and carries on its ventral side the large upper lip (oZ), 

 as well as the three typical pairs of Nauplius limbs (i, S, 3). The 

 anterior limb (first antenna) is uniramose, the next (second antenna)- 

 biramose and provided with a masticatory hook on the protopodite. 

 The mandible in CetocMlus is without such a masticatory process, 

 and is a simple biramose swimming limb. The limbs are beset at 

 their ends with long setae. 



In the alimentary canal we can distinguish a stomodaeum, a long 

 enteron, and a proctodaeum. The latter, in CetocMlus, in the first 

 NmqyUus stage, is still found as a solid ingrowth of the ectoderm, 

 the anal aperture not yet having broken through. The nervous 

 system is still connected for its whole course with the ectoderm. 

 As a sensory organ, we find the Nauplius eye. The coiled antennal 

 gland, which is probably developed at this stage, functions as^ 

 excretory organ, besides which the cells of the enteron seem 

 to have undertaken an excretory function ; urinary concretions- 

 have, at least, been proved to exist in certain projecting cells- 

 in the enteron of the Nauplius of Cyclops (Leydig, Fig. 89, ds, 

 p. 191). 



The terminal region of the Nauplius is sharply bent ventrally,. 

 and provided with two setae. We here find, internally on each 

 side, a large mesoderm-cell {ine) ; these are assumed by Grobben 

 to be primitive mesoderm-cells. 



Later Nauplius stages are distinguished by the greater length of 

 the body and by the outgrowth of its posterior region. During this 

 latter process, the more strongly chitinised integument of the dorsal 

 parts becomes marked off as the cephalo-thoracic shield by the de- 

 velopment of a fold at its margin. The proctodaeum has now become 

 perforated, a distinct dorsally-placed anal aperture being apparent. 

 The brain is connected behind the Nauplius eye with a paired 

 ectodermal growth, in which can be recognised the rudiments of 

 paired lateral eyes and their optic ganglia (secondary brain) ; these,, 

 at a later period, become vestigial. The rudiment of the genital 

 organs is to be recognised in large mesoderm-cells lying one on 

 each side of the alimentary canal (Fig. Ill B). 



There now appears, behind the mandible, a small biramose limb^ 

 the rudiment of the first maxilla (^), and the larva passes into the 

 first Metana^iplius stage (Fig. Ill B). 



A later Metanauplius stage (Fig. Ill (7) reveals three more rudi- 



