MALACOSTRACA 389 



An exceptionally large number of members of this class have direct 

 development. Of those which possess larvae only a few {Euphau- 

 siacea, a few of the Decapoda) hatch in the Nauplius stage. A special 

 characteristic of the larval development of the Malacostraca is the 

 occurrence of a zoaeal stage (p. 353), in which the carapace and tag- 

 mata are present, the abdomen is better developed than the hinder 

 part of the thorax, and the animal swims by biramous maxillipeds. 

 In crabs, hermit crabs, and some related families the Zoaea is 

 succeeded by a Metazoaea, which differs from it in having uni- 

 ramous rudiments of thoracic limbs behind the maxillipeds. In 

 other forms with larval development there is at this stage a prawn- 



au'. 



-l'ih.2 



thA- 



ab.G + t. 



th.8 

 ab.i)+(i+t. 



Fig. 266. Malacostracan larvae. A, Zoaea of Porcellana. B, Schizopod of 

 the lobster, C, Phyllosoma of Palinurus. D, Young Erichthus of a stomatopod. 

 ab. abdomen; an/ antennule; en. endopodite; ex. exopodite; t. telson; th. 

 thorax. The numerals indicate the somites or theirappendages. 



like Schizopod larva ('' Mysis'' stage), with biramous limbs on all 

 the thoracic somites, which is not always preceded by a Zoaea. 



The Malacostraca fall into two large groups and three smaller ones. 

 Of the latter, the Leptostraca retain, in the hinder end of the abdomen, 

 a primitive condition, which has been lost in the other groups. The 

 Stomatopoda (Hoplocarida) stand alone in possessing two free pseudo- 

 somites in the anterior part of the head, certain peculiarities of the 

 thoracic limbs, and peculiar gills oathe abdominal appendages. The 

 Syncarida unite certain features which are characteristic of other 

 groups. The large groups Peracarida and Eucarida contain most of the 

 members of the class. The former of these two divisions is character- 

 ized by possessing a brood pouch, formed by plates (oostegites) upon 

 the thoracic limbs, in which the young undergo a direct development. 



