iv PRACTICAL DIRECTIONS 293 



The animals above the Protozoa are placed, as we 

 have seen, in a number of different phyla, but as they 

 are all multicellular they are often spoken of collectively 

 as the Metazoa, one of the simplest of which we must 

 next examine. 



PRACTICAL DIRECTIONS 



Opalina. Make an incision in the rectum or small 

 intestine of a freshly-killed frog, and mount a little of its 

 contents in water on a slide. Having found an Opalina, 

 note its oval and flattened form, the uniform covering of 

 cilia, the cuticle, cortex, and medulla, and the absence of a 

 mouth and contractile vacuole. Stain with methyl-green 

 or magenta, and make out the numerous nuclei. Look out 

 for the products of division, free and encysted. 



Monocystis. Kill an earthworm by placing it for a few 

 minutes in methylated spirit. Then, with the scissors, slit 

 through the body-wall along the mid-dorsal line through 

 about the anterior third of the animal, when some white, 

 lobed bodies, the sperm-sacs (see p. 347) will be apparent. 

 Snip off a very small portion of one of these, place it on a 

 slide in salt solution and examine with the low power. 

 Amongst the cells of the sperm-sac you will probably be 

 able to distinguish : 



(a) Numerous cysts of Monocystis, many of which will be 

 seen to contain a number of spindle-shaped spores (Fig. 74, 

 E), and in some others, two gametocytes (B) ; (b) tropho- 

 zoites (A) in various stages of growth and contraction. 



Put on a high power, and after observing the groups of 

 cells of the sperm-sacs and sperms in various stages of 

 development, examine in detail : 



1. The trophozoites (A), noting the cuticle, cortex, medulla, 

 and nucleus. (Observe their movements.) 



2. The encysted gametocytes, before and after division into 

 gametes (B, C). 



3. Further stages in the formation of gametes and zygotes, 

 and the fully-developed spores (D, E). 



4. The structure of an individual spore, which contains 

 eight sporozoites (F). 



5. Sporozoites which have entered a clump of sperm-cells 

 (G); and later stages after they have become free, surrounded 

 by the tails of the sperms (H). 



Stain a preparation with methyl-green or magenta, and 

 go over i 5 again. 



