126 MORPHOLOGY OF INVERTEBRATE TYPES 



We shall consider them in their natural sequence. The append- 

 ages of the second (cephalic) somite are called the antennules 

 (or first pair of antennae). They are typically biramous and 

 consist of a two-jointed shaft or protopodite and two many- 

 jointed flagelli. The inner flagellum is the shorter one, the 

 outer flagellum bears many chemical setae or aesthetascs sup- 

 posedly of olfactory function. The first joint of the shaft con- 

 tains a statocyst with an opening to the outside. 



The appendages of the third somite are the antenna (called 

 also second pair of antennae), characterized by the exceedingly 

 long flagellum. They consist of a two- jointed protopodite, a 

 scale-like exopodite, and an endopodite or many-jointed flagellum. 

 The function of the antennas is tactile. The first joint of the 

 protopodite bears a papilla with the opening of the antennal or 

 green gland. The antennae are embryologically postoral ap- 

 pendages, but become soon preoral (prostomial) in position. 

 The appendages of the fourth somite are the powerful mandi- 

 bles. They consist of a single joint, probably homologous to 

 the coxopodite of a two-jointed shaft, and adapted to the 

 trituration of hard food; and of a three-jointed palpus repre- 

 senting the endopodite. This palpus is protected by a groove 

 in the mandible proper, in which it usually lies concealed. The 

 triturating edge of the mandible is formed by exceedingly hard 

 chitin. When the mandibles are removed from their sockets 

 in the process of dissection, the tendons of their muscles remain 

 adhering to them. 



The appendages of the fifth and sixth somite are the first and 

 second maxilla, respectively. Their function is chiefly to pass 

 on the food. They are quite thin, leaf-like structures. The 

 first maxilla consists of a two-jointed protopodite and an en- 

 dopodite with a short flagellum. In the second maxilla there 

 are present also an exopodite and an epipodite, while the en- 

 dopodite lacks the flagellum. Comparative study shows that 

 the epipodite is always derived from the exopodite. In the 

 second maxilla the epipodite is still short and wide. It func- 



