190 CRUSTACEA. PHYLLOCARIDA. 



MALACOSTRACA. 



The Malacostraca are of large or moderate size and 

 more highly organised than the Entomostraca. The num- 

 ber of segments is constant throughout the group, there 

 being five in the head, eight in the thorax, and six in the 

 abdomen ; each segment bears one pair of appendages. 

 In some cases the development is direct, in a few the 

 Nauplius stage occurs, but most commonly the Zoaea larva 

 is found. 



There are three Orders, (1) Leptostraca, (2) Thoracos- 

 traca, (3) Arthrostraca. 



ORDER. LEPTOSTRACA (PHYLLOCARIDA). 



This order forms a transition between the Phyllopoda 

 and the Malacostraca; in the Palaeozoic formations it is 

 represented by numerous forms, but only three genera are 

 now living, the most important being Nebalia. They are 

 provided with a large carapace which is usually bivalved 

 and covers the thoracic and some of the abdominal seg- 

 ments; in front it possesses a movable rostrum. There 

 are twenty-one segments in the body five in the head, 

 eight in the thorax, eight in the abdomen, the last 

 segment being forked or spined. There are nineteen 

 pairs of appendages, as in the Malacostraca: the head 

 bears two pairs of antennae, one of mandibles, two of 

 maxillae ; on the thorax there are eight pairs of leaf-like 

 feet similar to those of the Phyllopods ; the abdomen has 

 six pairs of appendages, the first four being large, the last 

 two small. The two posterior segments are without 

 appendages. The eyes are compound and stalked. 



