382 PHYLLOPODA. 



extremities. They are inserted at the sides of the large upper-lip 

 or labrum (I). Behind these are the posterior antenna?, which are 

 enormously developed and serve as the most important larval organs 

 of locomotion. They are biramous, being formed of a basal portion 

 with a strong hook-like bristle projecting from its inner side, an 

 inner unjointed branch with three bristles, and an outer large imper- 

 fectly five-jointed branch with five long lateral bristles. The hook- 

 like organ attached to this pair of appendages would seem to imply 

 that it served in some ancestral form as jaws (Glaus). This cha- 

 racter is apparently universal in the embryos of true Phyllopods, and 

 constantly occurs in the Copepoda, etc. 



The third pair of appendages or mandibles (md) is attached close 

 below the upper lip. They are as yet unprovided with cutting blades, 

 and terminate in two short branches, the inner with two and the outer 

 with three bristles. 



At the front of the head there is the typical unpaired eye. On 

 the dorsal surface there is already present a rudiment of the cephalic 

 shield, continuous anteriorly with the labrum (I) or upper lip, the 

 extraordinary size of which is characteristic of the larvae of Phyllopods. 

 The postcephalic region, which afterwards becomes the thorax and 

 abdomen, contains underneath the skin rudiments of the five anterior 

 thoracic segments and their appendages, and presents in this respect 

 an important variation from the typical Nauplius form. After the 

 first ecdysis the larva (fig. 208 B) loses its oval form, mainly owing to 

 the elongation of the hinder part of the body and the lateral exten- 

 sion of the cephalic shield, which moreover now completely covers over 

 the head and has begun to grow backwards so as to cover over the 

 thoracic region. At the second ecdysis there appears at its side a 

 rudimentary shell gland. In the cephalic region two small papillaa 

 (fr) are now present at the front of the head close to the unpaired 

 eye. They are of the nature of sense organs, and may be called the 

 frontal sense papillae. They have been shewn by Glaus to be of some 

 phylogenetic importance. The three pairs of Nauplius appendages 

 have not altered much, but a rudimentary cutting blade has grown 

 out from the basal joint of the mandible. A gland opening at the 

 base of the antennae is now present, which is probably equivalent to 

 the green gland often present in the Malacostraca. Behind the man- 

 dibles a pair of simple processes has appeared, which forms the rudi- 

 ment of the first pair of naaxillse (ma;). 



In the thoracic region more segments have been added posteriorly, 

 and the appendages of the three anterior segments are very distinctly 

 formed. The tail is distinctly forked. The heart is formed at the 

 second ecdysis, and then extends to the sixth thoracic segment: the 

 posterior chambers are successively added from before backwards. 



At the successive ecdyses which the larva undergoes new seg- 

 ments continue to be formed at the posterior end of the body, and 

 limbs arise on the segments already formed. These limbs probablv 

 represent the primitive form of an important type of Crustacean ap- 



