MALACOSTRACA 393 



Subclass PERACARIDA 



Malacostraca whose carapace, if present, does not fuse with more 

 than four thoracic somites ; whose eyes may be stalked or sessile ; and 

 which possess oostegites; a more or less elongate heart; and a few 

 simple coeca on the mid gut. 



A large subclass, containing several orders, which range from the 

 prawn-like Mysidacea, in which the caridoid facies (pp. 387, 388) is 

 practically intact, to the Isopoda and Amphipoda (slaters and sand- 

 hoppers) in which the carapace is lost and other features are greatly 

 modified. The important common characters which all these orders 

 possess are the presence of oostegites and the retention of the young, 

 which are directly developed, in a brood pouch formed by those 

 organs. Certain peculiarities, however, of the mandibles, which bear 

 behind the incisor process a movable structure known as the lacinia 

 viobilis, of the thoracic limbs (p. 336), etc., are also possessed in 

 common by the Peracarida. 



Order MYSIDACEA 



Peracarida with a carapace which covers most or all of the thoracic 

 somites; the eyes (when present), stalked; the scale of the antenna 

 well developed ; exopodites on most or all of the thoracic limbs, of 

 which one or two pairs are maxillipeds; and a well-formed tail fan. 



Small, usually pelagic crustaceans, most of which are marine, 

 though a few occur as "relicts" or immigrants in fresh waters. They 

 are mostly carnivorous, but take vegetable matter in the course of 

 feeding. Small food particles are obtained in a current set up by the 

 maxillae (p . 388) and when there are no gills also by a whirling action of 

 the thoracic exopodites, and are strained off by the maxillae: large 

 food masses are seized by the endopodites of the thoracic limbs. 



My sis (Figs. 265, 270), British, possesses a statocyst on the endo- 

 podite of each uropod, but has not the branched gills (thoracic 

 epipodites) which are found in some of the Mysidacea (Lophogas- 

 tridae). Its respiration takes place through the thin lining of the 

 carapace, under which a current is drawn from over the back by the 

 action of the epipodites of the maxillipeds (first pair of thoracic limbs). 



Order CUMACEA 



Peracarida with a carapace which covers only three or four thoracic 

 somites but is on each side inflated into a branchial chamber and pro- 

 duced in front of the head to lodge the expanded end of the exopodite 

 of the first thoracic limb; eyes (when present) sessile; no exopodite 

 on the antenna or endopodite on the maxilla ; three pairs of maxilli- 



