38 ZOOLOGICAL EXERCISES. 
matozoa will be found. In the female, notice in the 
ova the clear space called the germinal vesicle, which 
contains the germinal dot. In the male, notice the 
long filiform spermatozoa, with a very minute head 
at one end.* 
8. Cut off a small portion of the body-wall of the animal. 
Place it with the exterior side upward on a slide in 
water. With a brush, or by scraping with a knife, 
remove the outer skin—the ectoderm—and_ place 
portions of it on another slide. Place a cover-glass 
on the slide containing the portion of the body-wall, 
and with the handle of a needle press it firmly down, 
so that the preparation may be well squeezed out. 
Examine with the quarter inch, and by focussing 
up and down through the preparation, notice the two 
sets of muscular fibres crossing each other at right 
angles. The outer (upper in the preparation) fibres 
are more strongly marked, and lie transverse to the 
leneth of the column. The inner fibres are longi- 
tudinal, and are not so well marked. This muscular 
layer is the mesoderm of the animal. The endoderm 
is the internal layer of ciliated epithelium, and 1s 
difficult to make out. 
9. Take the slide with the fragments of skin, mount m 
water, cover, and examine with the quarter objective, 
notice the pigment spots, and thread cells. 
* If the student wishes to see moving spermatozoa, he must take a 
living animal, cut it in two, and examine the reproductive organs in sea 
water, In the same way he may see ciliary motion on the tentacles. — 
Fresh water destroys ciliary movement. 
