280 ZOOLOGY 
OrpeR 1. LEPTOSTRACA. 
CHARACTERISTICS.— This order includes but three recent genera: 
Nebalia, Paranebalia, and Nebaliopsis; they have bivalved 
shells, eight free thoracic segments with limbs of the Phyllopod 
type, and eight abdominal segments, with two caudal pro- 
CESSES. 
Nebalia forms an interesting transitional form between the 
primitive Phyllopoda on the one hand, and the Malacostraca 
on the other. It is of small size, with a laterally compressed 
body, partially enclosed in a bivalved cephalothorax which 
springs from the head. This cephalothorax extends over all 
the thoracic and four of the abdominal segments; it is closed 
by a special transverse muscle like that of the Ostracoda. 
The appendages are: 
1. Antennules. 5, 2nd pair of maxillae. 
2. Antennae. 6-13. 8 pairs of thoracic limbs. 
3. Mandibles. 14-19. 6 pairs of abdominal limbs. 
4, 1st pair of maxillae. 
The second antennae in the male, which is a smaller 
animal than the female, are produced into processes as long as 
the body. The mandibles bear a three-jointed palp, a structure 
not found in Phyllopods. The eight thoracic segments are 
very short from behind forwards; they each bear a pair of 
foliaceous appendages, which closely resemble the typical 
Phyllopod limb. Amongst these the female carries her eggs. 
The eight abdominal segments are considerably longer than 
the thoracic ; the first four bear large swimming-legs, consist- 
ing of a broad protopodite and an exopodite and endopodite ; 
the next two segments bear similar but much smaller append- 
ages, whilst the last two are limbless, so that the number of 
appendages is the typical Malacostracan nineteen, though there 
are twenty-one segments. The last segment bears two anal 
furcae, again an Entomostracan feature. 
Another primitive arrangement presented by Nebalia is 
the fact that the ventral nerve cord is to some extent con- 
tinuous with the hypodermis. In the abdomen the two cords 
run apart from one another, and are connected by transverse 
commissures. The eyes are stalked. 
The characteristic Malacostracan excretory gland is found 
