84 INVERTEBRATE ZOOLOGY. 



b. Trichina sjnralis. This is the famous parasite found in pork, causing 

 in those who eat it a dangerous muscular disease. It was tlae cause assigned 

 by Germany for prohibiting during several years the importation of American 

 pork. The parasite exists in infected meat in the encysted form, the separate 

 cysts appearing as minute white dots, somewhat oval in form, occurring be- 

 tween the separate fibres. Specimens of infected meat may often be obtained 

 from G-overnmental experiment stations or from private investigators. Tease 

 out a few scraps of the meat under the lens and look for the cysts. They are 

 better seen by reflected light. To study them they are best flattened gently, 

 and stained with borax carmine [42 b]. They should remain in the turpentine 

 until quite transparent, when the worms will be seen coiled up in the cysts. 



c. Nematode parasites in the frog. etc. Interesting parasitic forms may 

 be found in the lungs and intestines of the frogs, and generally appear while 

 searching for Trematodes [44]. The black lung-form is especially interesting, 

 being an hermaphroditic parasitic form, which alternates with a bi-sexual 

 form living in damp earth. This phenomenon is known as heterogony. (cf. 

 the text-books under Rliahdonema nigrovenosnm and Rhahditi.^ nigrovenosa.) 

 A little nematode found in the frog's intestine is very transparent and conve- 

 nient for microscopic stud}'. 



Type X.-Lumbricus Teirestris. 



60. Stitdij of a living specimen. Place a large earth-worm in a saucer 

 of water, and notice the following : — It elongates and contracts in length. 

 What sort of muscles cause this? What is the course of their fibres? It 

 changes its caliber at the same time. What sort of muscles do this ? What 

 difference in shape of the two ends? Which is anterior? Which end is the 

 more sensitive ? Where must the nervous system be best developed ? At 

 which end are the sense-organs best developed in all bilateral animals ? Why ? 

 Has the animal distinct dorsal, ventral and lateral aspects? Notice along the 

 sides, rows of minute bristles, especially prominent posteriorly. If the ani- 

 mal be drawn between the fingers, there is more resistance in one direction 

 than another. Can that be explained by the direction of the bristles ? Notice 

 the cinguluni, a fleshy girdle surrounding the body. Is this nearer the ante- 

 rior or the posterior end? Notice the body-rings or segment.'^. These are 

 true segments and not reduplicated abdomens as in the tape- worm. 



Q I . Killing and preserving. 



a. Killing. While dying, earth-worms are apt to contract and throw them- 

 selves into such tight coils that dissection isimi^ossible. To obviate this, place 



