1 1 8 MORPHOLOGY OF INVERTEBRATE TYPES 



carries four long plume-like bristles, one of which sits at the end 

 of the second joint and three at the end of the third joint. 

 The exopodite carries five similar bristles of which three sit at 

 the end of the last, in this case of the fourth joint. We have 

 seen already that these antennae are used for swimming. Con- 

 sequently they possess their own powerful muscles arising from 

 the back of the head and attached to the coxopodite. The third 

 pair of appendages are the mandibles. They are long chitinous 

 plates situated at the sides of the mouth and provided with a 

 masticating edge. The fourth pair of appendages are the 

 maxilla, two weak plates ending in four plume-like bristles. 

 The five pairs of thoracic limbs have a peculiar leaf-like ap- 

 pearance, with many bristles along their edge. The third pair 

 of limbs is the largest, the fourth next in size. The structure 

 as to detail differs in all five pairs, but the plan of structure re- 

 mains the same and may be best understood from the third or 

 largest limb. It consists of a large leaf-like, single-jointed pro- 

 topodite, by means of which it is attached to the body and the 

 edge of which is thickly covered with a row of long bristles; 

 a smaller, single-jointed exopodite with six plume-like bristles; 

 and a still smaller oval epipodite which serves as gill. The en- 

 dopodite is not developed and is represented only by a few 

 bristles situated between the bristles of the protopodite and those 

 of the exopodite. 



Digestive system. The mouth is situated on the head in the 

 depth of the depression which separates the head from the trunk. 

 At the sides of the mouth are two mandibles and below it two 

 maxillae. These two pairs of appendages are the masticating 

 organs of the animal. The alimentary canal is, as usual, com- 

 posed of three divisions, the foregut, the midgut, and the hindgut. 

 The foregut is represented by a short oesophagus which runs 

 slantingly upward and opens into the much wider midgut, 

 slightly projecting into it. The midgut is a cylindrical tube 

 running first upward, then straight backward, and finally down- 

 ward. Near the anterior end the midgut receives two dorsal 



