INVERTEBRATE ZOOLOGY. 95 



nient being constant in a given species, but varj-ing in different species. Fill 

 in the following blanks with the formula for CamharHs. indicating the pres- 

 ence of a well-developed gill by an X, a rudiment by R. and adding the 

 epipodites. 



G1I,L FORMULA FOR CAMBARUS. 

 Somite. Podo. Aut. Arth, Post. Arth. Pleuro. Epip. Total. 



VI 



VII 



VIII 



IX 



X 



XI 



XII 



XIII 



Total 



67. Disaection. of internal organs. Cambarus is dissected from the dor- 

 sal side as in other Arthropods. For an animal of this size, a small tin dis- 

 secting pan should be used, and the specimen covered wdth weak alcohol. A 

 lens may be removed from the dissecting microscope and held in the Angel's 

 over the object, when minute observation is required. If necessary, a small 

 and complicated part may be removed to a small glass dissecting pan. or 

 watch crystal, and examined microscopically. 



The specimen shoiild first be taken in the hand and the dorsal carapace care- 

 fully removed in small jiieces, taking gi-eat care not to harm the subjacent 

 organs. Then the abdominal tergites may be cut through at the sides and 

 removed. When this is completed, pin the specimen in the pan. 



[The student should now be ready to identify the different organs himself, at least in a 

 simple form like Cambarus, and hence general rules for identification of parts and order of 

 dissection will be substituted for the usual anatomical description. This method will be 

 followed as far as possible in the rest of the book, excepting, of course. Type XVIII, which 

 is designed as a preliminary study.] 



As a general rule, the aiinienfar!/ canal should first be sought, and when- 

 ever it is coiled or in an}' way obscures other parts, may be dissected away 

 from its connections and laid aside. In this case, however, the delicate cen- 

 tral organ of the circnlatory system, the " heart," lies above the intestine, and 

 should first be studied. The circulation can only be successfullv demonstra- 



