388 THE INVERTEBRATA 



respiratory epipodites. The abdominal appendages are biramous; 

 those of the first five pairs (pleopods) slender and fringed and used in 

 swimming, those of the last pair (uropods) broad, turned backward, 

 and forming with the telson a tail-fan, used in rapid backward move- 

 ment. There are no caudal rami. (The Leptostraca are the only 

 members of the class which possess these rami in the adult.) Food is 

 chiefly collected as particles in a stream which is set up by the 

 action of the maxillae and which passes forwards through a filtering 

 fringe of bristles upon the median margins of those appendages. 



This type is said to possess the caridoid fades . It is adapted prim- 

 arily to swimming and is best exhibited in the small, prawn-like, 

 pelagic forms, formerly classed together as Schizopoda but now dis- 

 tributed, as the orders Mysidacea (Fig. 265) and Euphausiacea, to the 



Fig. 265. A female oi Mysis relicta. After Sars. bd.p, brood pouch; 

 md.gr. mandibular groove ; sta. statocyst. 



two main subclasses of the Malacostraca (see below). Departures 

 from it are many and important, and most of its features have dis- 

 appeared more than once independently. Thus the carapace, the 

 inner ramus of the antennule, the scale of the antenna, the mandibular 

 palp, exopodites of thoracic limbs, etc., have been 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 Lepto- 

 straca 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, and when that happens 

 the animal ceases to gather food by filtration and adopts other modes 

 of feeding, for which its limbs, and particularly the thoracic endo- 

 podites, become variously modified — as, for instance, by the de- 

 velopment of chelae. 



