REVIEW OF GENUS ARGULUS — MEEHEAN 471 



anterior marginal groove on the carapace is distinguishable, and the 

 joint in the dorsal ribs is just discernible. 



On the first antennae the proximal segment is faintly defined and 

 has a spine projecting backward, while the third segment has de- 

 veloped a small process with two setae at the end. The palp on 

 the second antenna as well as the spine on the posterior margin of 

 the second segment has disappeared, but a seta has developed on the 

 anterior distal margin. The third segment is still small, the fourth 

 approximately the same as in the previous stage, while the fifth is 

 longest and ends in a number of long setae. Only a pair of post- 

 antennal spines remain where the transverse chitinous bar occurred 

 before the molt. 



The mandibular palp has disappeared and the chitinous framework 

 that connected it to the mandible is a simple longitudinal support, but 

 the junction of this rod to the mandible is still thickened into a spine, 

 which gradually disappears with each successive molt. A chitinous 

 thickening occurs on each side of the anterior transverse bar along the 

 buccal fold foreshadowing the serrated margin of the adult. The 

 small labial processes are clearly seen although the labial spines are 

 absent. There is a pair of spines in front of the mouth opening. 



The first maxillae consist of four segments by division of the basal 

 segment into two. The more distal of these has two spines on the 

 ventral proximal margin. A spine has developed on the posterior 

 margin of the second maxilla (the inner spine of the adult), and the 

 second segment has become somewhat longer than the more distal ones. 

 Spinules have made their appearance on the ventral side of the thorax, 

 and ramifications of the stomach have become slightly more complex. 



All the swimming appendages have assumed the shape of those on 

 the adult and serve as locomotor organs in the place of the antennal 

 and mandibular palps. A number of plumose setae have developed on 

 each. The abdomen is one-seventh to one-sixth as long as the body, 

 with the lobes projected slightly backward and spinules on th© 

 margins. 



Third stage. — ^When the larva has reached an age of five to seven 

 days a second molt occurs. The main difference noted between second, 

 third, and fourth stage larvae is in the pattern of the first maxillae, 

 the number of spines on the basal segment of the second maxillae, 

 small differences in size and number of setae and spinules, and in the 

 pattern of the branches of the stomach. 



The larva has reached a length of '1.0 to 1.2 mm. Behind the 

 nauplius eye the joint of the dorsal ribs is clearly visible, the posterior 

 transverse bar of the proboscis can be seen, and the buccal fold has a 

 process on its free margin. 



