182 HANDBOOK OF INVERTEBRATE ZOOLOGY. 



2. The antennules (Fig. 99) consist of a large, hairy, 

 basal joint (a), which is freely movable, and which carries 

 a large, two-jointed shaft (6), which ends in two small, 

 many-jointed flagella (c). 



Carefully disjoint the antennules, and examining the 

 inner surface of the basal joint, notice a longitudinal slit, 

 covered with hairs, and completely closed. This slit 

 marks the position of the external opening of the ear in 

 the young. In the adult crab it is closed, although it 

 remains permanently open in the lobster or crayfish. 



3. Outside the antennae are the large, movable, stalked, 

 compound eyes. Raise them up and disjoint them, and 

 place them with the other appendages. 



h. Study and compare the appendages. Each pair of 

 appendages is carried by a region of the body which 

 may, in certain Crustacea, be represented by a distinct 

 ring or somite, and a crustacean is therefore regarded 

 as consisting of as many somites as there are pairs of ap- 

 pendages. 



The series of somites and appendages is therefore as 

 follows : — 



1. Occular segment. Eyes. (This is not the proper 

 place for an examination of the question whether the eyes 

 are or are not homologous with the other appendages. I 

 have here followed the older writers, but not without 

 careful revision of the subject.) 



2. Antennulary somite. Antennules. Auditory Or- 



gans. 



3. Antennar}^ somite. Antennae. 



4. Mandibular somite. Mandibles. 



5. Metastoma. 



6. First maxillae. 



7. Second maxillae. 



