170 Illinois State Laboratory of Natural History. 



remaining gills arise from the posterior side, the branches 

 leaving the stem at approximately equal distances on 

 its basal portion, each branch being about as long as 

 the portion of the stem beyond their point of union 

 [Fig 4]. There are just one hundred of these filament- 

 ous gills. The head, prothorax, and terminal segment 

 are destitute of them, the mesothorax lacks one pair 

 and the penultimate segment lacks all hut one pair; the 

 remaining nine segments have the full number of ten to 

 each segment, five on each side, which Miiller* desig- 

 nates as follows; an anterior and a. posterior one form- 

 ing the suprastigmal pair, a similar infrastigmal pair, 

 and a pedal gill below these [Fig 5]. The number of 

 branches of the different filaments may be expressed by 

 a definite formula for each stage of larval life, notwith- 

 standing the fact that a slight variation from it is very 

 common, there being often one branch more or less in 

 one or all the filaments of a series. By studying a 

 number of individuals I endeavored to eliminate this 

 variation, and the branches in a full grown larva may 

 be tabulated accordingly as follows, the segments back 

 of the head being numbered consecutively: 



*Zool. Jahrbuch. Ab. Syst, VI., p. 6-*C. 



i a., anterior: p., posterior; s., suprastigmal; i., infrastigmal; ped., pedal. 



