4 WRIGHT AND MACALLUM. [Vol. I. 



year, when the laboratory aquarium was fully stocked with full- 

 grown Necturi. It was soon observed that these would furnish 

 abundant material for a thorough study of the organization of 

 Sphyranura, and this was accordingly undertaken, with the 

 results given in the following pages. 



Methods of Sticdy. 



Specimens of Sphyranura are rarely too large to prevent a 

 complete study in the fresh condition. The weight of a thin 

 cover-glass is alone necessary to render the whole body accessi- 

 ble to high powers. Of course the opacity of the yolk-glands 

 interferes with the examination of the nerve cords, and some of 

 their commissures ; but these glands are so little developed in 

 young examples as to offer no obstruction. Without abundance 

 of fresh material many points, such as the ciliation of the 

 water-vascular system, its openings, the course of the genital 

 products within the ducts, the presence of tactile hairs, and of 

 the smaller nerve-commissures, would have been overlooked or 

 remained obscure. The thorough study of certain of the 

 cellular elements in the living condition likewise perm.itted the 

 selection of the fixing reagent, which gave us the nearest 

 approach to the natural condition of the cells. This precaution 

 appeared to us particularly desirable, as some of the more 

 important recent memoirs dealing with the anatomy of 

 Trematodes are based upon material preserved in alcohol and 

 other inadequate fixative reagents. With such delicate elements 

 as are to be found in the Trematodes the utmost care is 

 required in all the preliminary operations of microtomy to 

 obviate shrinkage and alteration of the cell-structure. Various 

 fluids which have been recently recommended by histologists 

 for fixation were experimented with, and it was found that here, 

 as in higher animals, no one fluid is equally successful with all 

 tissues. It was necessary to select that giving the best general 

 results, and the fluid which satisfies this requirement is one 

 which, as far as we are aware, has hitherto only rarely been 

 employed for the study of invertebrate tissues, viz., Flemming's 

 chrom-osmio-acctic mixture. 



The following is the method of procedure employed: an 

 example of Sphyranura is placed on a slide with merely suffi- 

 cient water to cover it ; a drop of the reagent is placed beside 



