'214 H. OHAIIAM CANNON 



lilio glands could bo separated into two groups, tlic lirst group 

 lying under the brain and over the oesophagus and the second 

 grou}) consisting of very large cells lying nearer the tip of the 

 lal)runi. The first group sent out a long thin efferent duct 

 which, after making many twists, allowed the exit of the 

 secretion in front of the mouth. Cunnington's description 

 differs essentially from this in that he could not obserAC a duct 

 from the first group but did observe an efferent duct from the 

 second group. Cunnington also distinguishes two groups of 

 cells — a proximal group of several small cells and a distal 

 group of large cells. The proximal group, he states, lie close 

 against the chitinous cuticle and are obviously modified 

 epidermal cells and possibly act as replacement cells, taking 

 the place of cells in the distal group when these lose their 

 secretory power. The latter group usually consists of four 

 colls only and these are placed one behind the other, the most 

 extreme possessing a duct opening on the inner side of the 

 labrum. They have characteristic nuclei, which are shaped like 

 a hollow bowl and thus appear circular or semi-circular in 

 section. The secretion is formed in the neighbourhood of the 

 nuclei in the form of little drops which fuse to larger drops 

 or rods or bands and pass to the exterior. Cunnington suggests 

 that the duct of the extreme cell of the distal group acts as 

 a connnon duct for the whole group. 



Methods. 



For Simocephalus vetulus the best fixative was 

 found to be cold saturated sublimate in distilled water. This 

 gave excellent fixation and did not produce distortion as did 

 most other fixatives. Good results were also obtained with 

 a mixture of equal parts of saturated sublimate in distilled 

 water and 1 per cent, osmic acid. This mixture, a modifica- 

 tion of Mann's fixative, was allowed to act for about an hour. 

 In comparing Simocephalus with other Cladocera, it was 

 found that for Daphnia the best results were obtained with 

 subhmate acetic acid, while for Graptolebris and Camp- 

 tocercus Carnoy gave the best fixation. A young Chiro- 



