EXTERNAL CHARACTERS. 35 



canal of another, with a thin injection, such as freshly preci- 

 pitated Prussian blue, suspended in water. Inject the coloured 

 fluid, by means of a very fine glass cannula provided with an 

 indiarubber cap, into the hinder end of the animal; in the 

 middle line for the excretory system, and about 1 mm. from it for 

 the alimentary canal. Place the animals between glass slips, 

 and tie the slips together with cotton. Put the slips with the 

 animals between them into alcohol (90 per cent.) for twelve hours 

 or more, when the specimens may be dehydrated, cleared, and 

 mounted in balsam. 



2. Squeeze specimen somewhat more lightly between two 

 glass slides, and leave it in corrosive acetic mixture (see Appendix) 

 for five or six hours. Wash well with 50 per cent., 70 per cent., 

 80 per cent, alcohol for about two days. Pass gradually down 

 to water, and stain with weak hcemalumfor twelve hours. Wash 

 for the same length of time in 1 per cent, alum solution. Then 

 pass gradually through water, 50 per cent., 70 per cent., 90 per 

 cent., to absolute alcohol (two hours). Clear in cedar-wood 

 oil, and mount in balsam. 



A. External Characters. 



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



angular projection forming its 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 



line, near the junction of the triangular anterior 

 portion 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, pointed scales, which are best seen in a 

 specimen kept in spirit and dried at the moment of 

 examination. 



