MALACOSTRACA 349 



lost in various branches of the malacostracan tree. Only the number 

 of the somites and the size of the tagmata are constant, save in the 

 case of the Leptostraca already mentioned and in certain parasitic 

 isopods. Departure from the caridoid facies is associated with the 

 abandonment of the swimming habit for crawling or burrowing. 



An exceptionally large number of members of this group have 

 direct development. Of those which possess larvae only a few 

 (Euphausiacea, 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. 316), in which the tagmata are 

 present, the abdomen is better developed than the thorax, and the 

 animal swims by biramous maxillipeds. 



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

 Of the \a.ttery 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 

 pseudosomites in the anterior part of the head, certain peculiarities 

 of the thoracic limbs, and peculiar gills on the 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 characterized by possessing a brood pouch, formed by plates {ooste- 

 gites) upon the thoracic limbs, in which the young undergo a direct 

 development, and by the freedom of some or all of the thoracic 

 somites from the carapace. The Eucarida do not possess a brood 

 pouch and usually have larval stages, their heart is a short chamber 

 in the thorax, and their carapace fuses with the dorsal side of each 

 thoracic somite. 



Subclass LEPTOSTRACA 



Malacostraca with a large carapace provided with an adductor muscle 

 and not fused with any of the thoracic somites; stalked eyes; the 

 thoracic limbs all alike, without oostegites, biramous, and usually 

 foliaceous; seven abdominal somites, of which the last bears no 

 appendages; and caudal rami on the telson. 



Nebalia (Fig. 254) is the commonest and typical genus of this 

 group. N. bipes, the British species, may be found between tide- 

 marks, under stones, especially in spots which are foul with organic 

 remains. Nebalia has a rostrum, which is jointed to the head. The 

 antennae have no scale, while the antennules are unique in possessing 

 one. The carapace encloses the four anterior abdominal somites. The 

 thorax is short. Its limbs (Fig. 211 E) are flat. Their endopodite is ' 

 narrow and possesses five indistinct joints. Sometimes the long 

 basipodite is divided and its distal region added to the endopodite 



