I. CRUSTACEA: SCHIZOPODA 



375 



Legion I. Leplostraca. 



The Leptostraca connect the Phyllopoda with the higher groups. Their 

 twenty-one somites (eight abdominal, eight thoracic, and five cephaUc), and the 

 openings of the genital ducts ally them to the Malacostraca. On the other 

 hand, the bivalve carapace covering the cephalothorax and part of the abdomen, 

 and the leaf-lilve thoracic feet, are phyllopodan. They have an antennal gland 

 and a rudimentary shell gland; an elongate heart whicli extends through 

 cephalothorax and abdomen; and stalked compound eyes. The few species 

 are all marine and belong to the genus Nebalia* (fig. 395). 



Fig. 395. — Nebalia bipes* (after Sars). h, heart; i, intestine; 0, ovary; a, adductor 

 of carapace; b, brain; r, rostrum. 



Legion IL Tlwracostraca {Podoplithalmia). 



The names given this division have reference, first, to the fact that the 

 head and some of the thoracic segments are immovably united and covered 

 by a firm carapace; second, that the compound eyes (except in Cumacea) 

 are placed on movable eye stalks. The first five appendages are always 

 two pairs of antennae, a pair of mandibles, and two pairs of maxilla;. 

 The remaining pairs vary greatly and from one to three may be modified 

 into maxillipeds, while the abdominal somites except the last {telson) 

 usually bear appendages, at least in the female. There is usually a 

 metamorphosis in which a nauplius stage may appear, most frequently 

 in the lower forms (schdzopods), but even in the decapods (Peneus). 



Order I. Schizopoda. 



These are small forms (fig. 396), mostly marine, in which the cephalothorax 

 is covered by a carapace with which some or all of the thoracic somites are 

 firmly united. The eight thoracic feet are biramous throughout life and are 

 used in swimming. The posterior pair of abdominal feet together with the 

 telson form a caudal 'fin' by means of which the animal can swim backwards. 

 The delicate skin permits of diffuse respiration, and gills are frequently lacking. 

 In some genera plates from the legs of the female enclose a brood case beneath 

 the cephalothorax, thus giving these forms the common name of opossum 

 shrimps. 



