CRUSTACEA. 



381 



The term has now been extended to a very large number of larvae 

 which have certain definite characters in common. They are provided 

 (fig. 208 A) with three pairs of appendages, the future two pairs of 

 antennas and mandibles. The first pair of antennas (an 1 ) is uni- 

 ramous and mainly sensory in function, the second pair of antennas (a??- 2 ) 

 and mandibles (md) are biramous swimming appendages, and the 

 mandibles are without the future cutting blade. The Nauplius 

 mandibles represent in fact the palp. The two posterior appendages 

 are both provided with hook-like prominences on their basal joints, 

 used in mastication. The body in most cases is unsegmented, 

 and bears anteriorly a single median eye. There is a large upper lip, 

 and an alimentary canal formed of oesophagus, stomach and rectum. 

 The anus opens near the hind end of the body. On the dorsal sur- 

 face small folds of skin frequently represent the commencement of 

 a dorsal shield. One very striking peculiarity of the Nauplius accord- 

 ing to Glaus and Dohrn is the fact that the second pair of antennae 

 is innervated from a sub-oesophageal ganglion. A larval form with 

 the above characters occurs with more or less frequency in all the 

 Crustacean groups. In most instances it does not exactly conform 

 to the above type, and the divergences are more considerable in the 

 Phyllopods than in most other groups. Its characters in each case 

 are described in the sequel. 



Phyllopoda. For the Phyllopoda the development of Apus can- 

 criformis may conveniently be taken as type (Glaus, No. 454). The 

 embryo at the time it leaves the egg (fig. 208 A) is somewhat oval in 

 outline, and narrowed posteriorly. There is a slight V-shaped inden- 

 tation behind, at the apex of which is situated the anus. The body, 

 unlike that of the typical Nauplius, is already divided into two regions, 

 a cephalic and post-cephalic. On the ventral side of the cephalic 

 region there are present the three normal pairs of appendages. 

 Foremost there are the small anterior antennas (an 1 ), which are 

 simple unjointed rod-like bodies with two moveable hairs at their 



A an 



FIG. 208. Two STAGES IN THE DEVELOPMENT OF APUS CANCBIFORMIS. 



(After Claus.) 



A. Nauplius stage at the time of hatching. 



B. Stage after first ecdysis. 



an 1 , and an 2 . First and second anteunte; md. mandible; mx. maxilla; I. labrum: 

 fr. frontal sense organ ; /. caudal fork; s. segments. 



