348 THE INVERTEBRATA 



additional somite is present, but in the adult it has fused with that 

 which precedes it.) The female openings are always on the 6th thoracic 

 somite, and the male on the 8th. A carapace encloses the thorax at 

 the sides. The median eye is vestigial in the adult, and the compound 

 eyes stalked. The antennules arebiramous, as they are in no crustacean 

 of any other group. The antennae have a scale-like exopodite. The 

 mandibles have uniramous palps and the part which projects to- 

 wards the mouth is cleft into "incisor" and "molar" processes. 

 The maxillules have two endites (on the first and third joints) 

 and the maxillae four, grouped in twos. The thoracic limbs have 

 a cylindrical, five-jointed endopodite (p. 300), a natatory exopodite, 

 and two 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 back- 



Fig. 253. A female of Mysts relicta. After Sars. bd.p. brood pouch; 

 md.gr. mandibular groove; sta. statocyst. 



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

 movement. There are no caudal rami. (The Leptostraca are the 

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

 Food is collected as particles in a stream set up by the action of the 

 maxillae, which also bear the filtering fringes of bristles. 



This type is said to possess the caridoid fades . It is best exhibited 

 in the small, prawn-like, pelagic forms, formerly classed together as 

 Schisopoda but now distributed, as the orders Mysidacea (Fig. 253) 

 and Euphausiacea, to the two main subclasses of the Malacostraca 

 (see below). Departures from it are many and important, and most of 

 its features have disappeared 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 



