ANIMAL PARASITES 75 



make thorn important objects of study. Among the most widely 

 distributed and best known forms are Monocystis agilis and 

 related species from the seminal vesicles of earthworms. In 

 many localities practically every worm is infested and all stages 

 may be found. 



PRACTICAL WORK 



Directions for Dissection. — A fresh specimen of Lumbricus 

 terrestris or related species of earthworm will be supplied. Place 

 the worm in a flat-bottomed tray with the dorsal surface upward, 

 stretching with a pin through the fifth segment and in the 

 neighborhood of the thickened clitellum. Very carefully slit 

 through the skin along the mid-dorsal line, avoiding piercing 

 the alimentary canal or other organs. The conspicuous cream- 

 colored bodies at the side of the oesophagus, and overlying it in the 

 ninth, tenth, and eleventh segments are the seminal vesicles. 



Remove with forceps a bit of one of the seminal vesicles and 

 tap it in a drop of physiological salt solution (or water) on a 

 slide. Cover the preparation and observe and make careful 

 drawings of the following: 



Normal Tissues. — Preparatory to study of the parasitic 

 organisms, distinguish and make drawings of the following normal 

 tissues of the seminal vesicle: (1) the sperm morula?, spherical 

 masses of small rounded cells which should be studied both in 

 surface view and in optical section; (2) mature spermatozoa in 

 brush-like clusters attached to a central core of protoplasm; 

 (3) epithelial cells, flattened, angular, and homogeneous. 



Monocystis sp. — Having distinguished the normal tissues 

 in your preparation, search out various stages of the gregarine 

 parasites. Study and figure the stages as you find them, but 

 arrange your drawings in the following order: 



1. The young trophozoite, or feeding stage, is a minute nucleated 

 body in the midst of a sperm morula. As it develops at the 

 expense of the sperm cells it becomes elongated and finally 

 appears as a spindle-shaped organism with a cilia-like covering 

 of short, abortive spermatozoa. 



2. Motile trophozoites, extended and contracted, may be 

 found in the seminal fluid. The living organism exhibits slow, 

 flowing movements. Note the clear ectosarc, granular endosarc, 

 and nucleus. 



