EXTERNAL CHAEACTEES 27 



aUmentary canal of another, with a thin injection such as 

 finely-powdered carmine, or freshly-precipitated Prussian 

 blue, suspended in water. To do this make a very small 

 incision with a sharp-pointed knife near the hinder end of 

 the ammal — in the middle line for the excretory system, and 

 about 1 mm. from it for the alimentary canal. Inject the 

 colov/red fluid into the opening by means of a very fine glass 

 cannula with an indiarubber cap. Place the animals, still 

 aUve, between glass slips, and tie the slips together with 

 cotton, so as to slightly flatten the animals. Put the slips 

 with the animals between them into alcohol (90 per cent.) for 

 twelve hours or more, when the animals may be dehydrated, 

 cleared, and mounted in balsam. 



§ 2. Squeeze a third specimen somewhat more tightly 

 between two slips, and leave it in alcohol (90 per cent.) 

 for twenty-four hours. Stain it faintly with weak borax- 

 ca/rmine, and mount it in the ordinary way. 



A. External Characters. 



1. In form the animal is flat and oval, with a blunt tri- 



angular projection from its broad anterior end. 

 Its length is about an inch and a half, its breadth 

 about half an inch. 



2. The mouth is an oval aperture at the anterior end, in 



the middle of the cup-shaped anterior sucker. 



3. The ventral sucker is a muscular cup in the mid- ventral 



hne, near the junction of the triangular anterior por- 

 tion with the broader part of the body. 



4. The genital aperture, through which the penis may be 



protruded, is on the ventral surface between the two 

 suckers, and slightly nearer the posterior one. 



5. The cuticle is a thin layer covering the whole animal. 



Its surface is covered with minute backwardly-directed 

 spines, which are best seen in a specimen kept in 

 spirit, and dried at the moment of examination. 



B. The Alimentary Canal. 



1. The pharynx is an oval muscular mass immediately 

 behind the mouth. 



