Xll] LEPTOSTRACA 215 



that the carapace, which as we have seen is an outgrowth of the head, 

 has become fused with the dorsal surface of the thoracic segments, 

 whilst at the sides it forms freely projecting flaps, which since they 

 cover the gills are known as branchiostegites (Gr. oreyo), to cover). 

 The excretory organ of most of the Malacostraca opens at the 

 base of the second antennae and not as in the Entomostraca on the 

 second maxilla. As a rule the typical larva the Nauplius of the 

 last-named group is not present in the life-history of the Malac- 

 ostraca, which may hatch out from the egg in a practically adult 

 condition or may pass through several larval stages, the first of 

 which is the Zoaea, a larva with many appendages, possessing eyes 

 and in all ways more differentiated than the Nauplius. 



Order I. Leptostraca. 



The order Leptostraca (Gr. ACTI-TOS, slight, small) contains but 

 three genera, which are interesting because they form an inter- 

 mediate stage between the Malacostraca and the Entomostraca. 

 Like many of the latter they are provided with a bivalve carapace 

 which, unlike that of all other Malacostraca, is not fused with the 

 thoracic segments. Behind the six appendage-bearing segments of 

 the abdomen there come two more segments without limbs and 

 the hindermost bears two diverging filaments constituting a " caudal 

 fork," such as is. commonly found amongst the Entomostraca. The 

 thoracic limbs are flattened and leaf-like, as in Apus, but the 

 mandible bears a three-jointed feeler or palp and the eyes are 

 stalked, both, on the whole, Malacostracan characters. The 

 excretory organ opens on the second antenna, but in the larva the 

 shell-gland or maxillary excretory organ is found and traces of it 

 exist in the adult. 



The order is marine, and very widely distributed throughout the 

 ocean. Its members are capable of living and thriving in very foul 

 water, so foul as to be fatal to most other animals. Nebalia is 

 the best known genus. 



Order II. Thoracostraca. 



The Thoracostraca (Gr.' 0<o'pa, a breast-plate) form a large 

 group and contain many very different forms. They are placed 

 together because the carapace has become fused with several of the 

 thoracic segments, so as to form a region known as the cephalo- 

 thorax, the exoskeleton covering which is not jointed and is not 

 bivalved as in Nebalia. The eyes of the Thoracostraca are 

 compound and almost always are borne on movable stalks. The 

 order is divided into four sub-orders. 



