428 



ARTEROPODA. 



Order I. Schizopoda. 



These are small forms, mostly marine, in which the cephalo- 

 thorax is covered by a carapace with which some or all of the 



FIG. 433. Amphithoe. (From Gerstacker.) a", a 2 , first and second antennae ; aw, eye; 

 VlI-XIIIi the seven free thoracic segments ; 1-7, abdominal segments. 



thoracic somites are firmly united. The eight thoracic feet retain 

 throughout life a biramous condition and are used in swimming. 

 The posterior pair of abdominal feet together with the telson form 



FIG. 434. Mi/sis elonqata. (From Gerstacker.) a, /3, first and second antennae; a, ex- 

 pedite; cm, eye; , endopodite; o, otocyst ; 1-7, abdominal somites. 



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

 The delicate skin permits of diffuse respiration, and gills are fre- 

 quently 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. 



