32 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. 
