CRUSTACEA. 461 



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 

 cancriformis may conveniently be taken as type (Claus, No. 454). 

 The embryo at the time it leaves the egg (fig. 208 A) is some- 

 what oval in outline, and narrowed posteriorly. There is a 

 slight V-shaped indentation 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 antennae (an 1 ), which are simple 

 unjointed rod-like bodies with two moveable hairs at their 

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

 lip or labrum (/). Behind these are the posterior antennae, 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 imperfectly 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 (Claus). This character 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 (/) or 

 upper lip, the extraordinary size of which is characteristic of the 

 larvae of Phyllopods. The post-cephalic 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 



