26 INVERTEBRATE ZOOLOGY 



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 relations 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 

 appendages. All of these appendages, except the first pair, 

 however much they may differ from one another, are modifi- 

 cations 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 inner or 

 endopodite, and the outer 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, and is sometimes called a uropod, but yet has the 

 typical parts. Label its parts. 



Exercise 6. (a) If the animal be a male, remove and draw the 

 right-hand first and second swimmerets. These are modi- 

 fied from the typical structure to serve as copulatory 

 organs. 



