24 PHYLUM ARTHROPODA 



Exercise 1, Draw an outline of the dorsal side of the animal and 

 label all the parts described above. 



Cut off the right branchiostegite with the scissors, taking care 

 not to injure the gills beneath. Push aside the gills and notice the 

 thin integument which forms the lateral wall of the cephalo- 

 thorax. Observe the method of attachment of the gills. They are 

 feathery, thin-walled expansions of the body wall and are at- 

 tached either to it or to the basal portions of the legs. They 

 present a very large surface to the surrounding water, and the 

 blood circulating through them is thus oxygenated. Notice the 

 epipodites, the skinny flaps which project from the basal joints of 

 many of the legs and separate the gills of a segment from those of 

 the next. They are not prominent in the crayfish. 



Exercise 2. Without displacing the gills or epipodites make a sketch 

 of them as they lie in the gill chamber. 



Exercise 3. Draw a diagram representing an ideal transverse section 

 of the body wall in the region of the walking legs ; show the rela- 

 tions of the branchiostegites, the legs, and the gills to the body. 



The Appendages. Of these there are nineteen pairs, each somite 

 of the body, with the exception of the last one, bearing a pair. 

 There are thus thirteen cephalothoracic appendages, of which 

 five are cephalic and eight thoracic, and six abdominal append- 

 ages. All these appendages, except the first pair, however much 

 they may differ from one another, are modifications of a single 

 primitive type of structure. This type has been least modified in 

 certain of the abdominal appendages. We shall, consequently, 

 study these first. 



Exercise 4. The abdominal appendages are called swimmerets or 

 pleopods. Cut off the right swimmeret of the fourth abdominal 

 somite close to the body, draw it on a large scale, and label all its 

 parts. It consists of a basal piece, the protopodite, and two terminal 

 branches, the medial, or endopodite, and the lateral, or exopodite. 

 This type of structure is characteristic of all crustacean appendages 

 except the pair belonging to the first somite; those appendages 

 which apparently differ from this type are modifications of it. 



Exercise 5. Remove and draw on a large scale the right-hand 

 sixth swimmeret. It is quite different from the last one drawn. 



