A CRAYFISH OR A LOBSTER 



27 



Write opposite the number of each somite, in your notebook, 

 first, the name of the appendage belonging to it, then indicate 

 what parts that appendage possesses by a '' + " and what parts 

 it lacks by a ^^ — " under the appropriate head, as is shown in 

 the case of the sixth somite in the table on page 26. 



The Gills. Remove the left-hand branchiostegite. Place the 

 animal in water and study the gills on the left side. These organs 

 may occur on the eight thoracic somites, and on each segment 

 they may be attached either to the basal joint of the leg, when 

 they are called podobranchiae, to the flexible joint by which the 

 leg articulates with the body, when they are called arthro- 

 branchise, or to the body wall just above the leg, when they are 

 called pleurobranchiae. A single thoracic somite may bear on 

 each side four gills, — a pleurobranch, two arthrobranchs, an an- 

 terior and a posterior, and a podobranch, — but on most of the 

 somites a less number is present. 



Exercise 18. Construct in your notebook a table showing the ar- 

 rangement and number of the gills and also of the epipodites and 

 their relations to the somites bearing them, as follows : 



Begin with somite 13 and indicate by a '' + " under the proper 

 head opposite the number of each somite the presence of the gill 

 or epipodite, and by a '' — " its absence. 



The Internal Organs. With strong scissors and forceps care- 

 fully remove the shell from the entire dorsal surface of the animal, 

 taking great care not to disturb the organs lying beneath. Notice 

 just beneath the shell a pigmented membrane. This is the under- 

 skin, or hypodermis, the matrix of the shell. Entirely remove the 



