216 ARTHROPOD A [CH. 



Sub-order 1. Schizopoda. 



This sub-order includes the lowest of the Thoracostraca. The 

 name is suggested by the circumstance that all the eight pairs of 

 thoracic limbs are biramous ; the first and sometimes the second 

 pair are reduced in size arid provided with gnathobases ; they assist 

 the mandibles and maxillae and hence are termed maxillipedes. It 

 will occur to most observers that the thoracic feet of the Schizopod 

 resemble the ordinary form of swimmeret or abdominal appendages 

 in the more familiar Lobster or Crayfish. This is so ; the swim- 

 merets of a Schizopod are however sharply distinguished from 

 the thoracic limbs by their smaller size. It appears probable 

 that the first step in the evolution of an abdomen was the 

 reduction in size of the appendages so as to transform the hinder 



FIG. 84. Nyctiphanes norwegica, a Schizopod. Slightly magnified. From 

 Watase\ The black dots indicate the phosphorescent organs. The gills 

 are seen between the cephalothoracic and the abdominal appendages. 



part of the body into a powerful steering fin or rudder, and many 

 Schizopods only use the abdomen in this way, since most of the 

 swimmerets are very small and appear to be practically function less. 

 The last one however is broad and assists the tail in its vigorous 

 strokes. Some Schizopods have a series of phosphorescent organs 

 which under certain conditions emit a pale but very perceptible 

 light like that of a glow-worm. This light seems to be controlled 

 by the animal but its use is not very clear. 



There are very interesting differences amongst the genera com- 

 posing the Schizopoda. The genus Euphausia for instance has 

 long feathery gills attached to the basal joints of the thoracic legs 

 and the eggs are not borne about by the mother but hatch out into 

 Nauplii, which pass through a series of metamorphoses before 

 becoming adult. In Mysis on the other hand the gills are few and 

 simple and the eggs are borne under the thorax on flat plates 

 termed oostegites, which project inwards from the hinder thoracic 



