HIGH SCHOOL ZOOLOGY. ^-^ 193 



grouped in three regions, of which the head contains five, the 

 thorax eis;ht and the abdomen seven. On account of the ex- 

 tent to which a segment from one region difiers from that from 

 another, the segmentation is styled heteronomous, but the same 

 fundamental plan of structure may be observed in all. The 

 abdominal segments are independent, but the segments of the 

 head and tliorax are coalesced with each other into a cephalo- 

 thorax, the fusion being moi-e complete on the dorsal surface. 

 Behind a line which marks off the cephalothorax into anterior 

 and posterior regions, each side of the thorax is provided with 

 a flap of skin which acts as a gill-cover, forming a cavity in 

 which the gills, attached to the bases of the thoracic legs, are 

 sheltered. 



6, It will be convenient to study one of the hinder pairs of 

 abdominal appendages first ; they are biramous, consisting of 

 a basal part, with two branches, internal {endojiodite) and ex- 

 ternal {exoj)odite). Those of the sixth pair are modified with 

 the telson into the caudal fin ; while the first and second pairs 

 are difierent in the two sexes. 



Of the eight pairs of thoracic legs, the three foremost are 

 turned forward as the foot-jaws (maxillij^edes) to assist in 

 securing food, while the five hindmost are the walking legs. 

 Comparing these with the abdominal appendages, we find that 

 although the endopodite is large in all, the exopodite is only 

 present in the foot-jaws, while, with the exception of the 

 eighth pair, all the thoracic appendages have in addition a 

 membranous flap — the epipodite — concealed within the gill- 

 chamber, and carrymg, with the exception of the first, gill- 

 filaments. There are thus six gills of this nature on each side ; 

 the other gills are attached to the soft membrane which con- 

 nects the legs to the thorax, and there are eleven of these on 

 each side, the third to the seventh appendages each carrying 

 two, while the second has only one. The fourth pair of the- 



