28o ZOOLOGY 



Order 1. LEPTOSTEACA. 



Characteeistics. — This order includes but three recent genera: 

 Neialia, Paranebalia, and Nebaliopsis ; they liave hivalved 

 shells, eight free thoracic segments with limbs of the Phyll&pod 

 type, and eight abdominal segments, with two caicdal 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. Autennules. 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 structxire 

 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 



