3OO CHARLES ALBERT SHULL. 



was found to have three pairs of appendages instead of two, mod- 

 ified for sexual purposes. . The right swimmeret of the pair is 

 shown in Fig. 3, the left one being exactly like it. The charac- 

 ter of the modification in this third pair is just the same as that of 

 the second, except that the projection of the endopodite is smaller 

 than on the second. The first and second pairs of appendages 

 are perfectly normal. Moenkhaus, '03, has reported an exactly 

 similar case in the same species, and so far as I know these are 

 the only two cases on record. 



THE PROBLEM, MATERIALS, AND METHODS. 



Being convinced that the abnormal swimmerets of the female 

 C. virilis described herein were the result of regeneration after a 

 somewhat extensive injury, and having before me the work of 

 Morgan on Eupagurus longicarpus, and of Miss Steele on C. virilis 

 and C. gracilis, I determined to perform a series of experiments 

 with the crayfish in order to ascertain whether or not the ab- 

 dominal swimmerets regenerate, and if so, under what conditions. 

 These experiments were begun in 1905 ; and after two years of 

 rather unsuccessful work the choice of material and the methods 

 of handling it were so improved that gratifying results have been 

 obtained. 



At first attempts were made to keep the crayfishes in aquaria 

 of running water. A large aquarium was divided by partitions 

 of galvanized iron netting into a number of compartments, each 

 about 30 cm. square. Each compartment was provided with 

 gravel and flat stones, and fresh water from Lake Michigan was 

 kept running constantly at a depth of about 5 cm. 



The aquarium was cleaned frequently, and all precautions were 

 taken to keep the animals in sanitary conditions ; but in spite of 

 the great care exercised, the crayfishes would die after several 

 weeks of confinement. Since my first material was adult, and 

 moulted therefore infrequently, death took place before any 

 moults occurred. Many variations of these conditions were tried 

 in an attempt to secure more favorable results, but without 

 success. 



During the last two years a method has been employed, which 

 has obviated all difficulties, and has given entirely satisfactory 



