OTHER NEMATODES 107 



a ductus deferens (vas deferens), a portion of which is expanded as 

 a seminal vesicle serving for temporary storage of spermatozoa. This 

 is followed by a short muscular region, the ejaculatory duct, which 

 opens into the rectum. The penial setae are also considered to be a 

 part of the male reproductive system. The female reproductive system 

 consists of paired ovaries, each of which is continuous with an oviduct 

 which expands to form a uterus. The two uteri unite to form a single 

 vagina which opens to the outside by way of the genital pore pre- 

 viously seen. Understand from textbook descriptions the reproduction 

 and the life-cycle in Ascaris. 



Exercise 3. — The Structure as Seen in Cross Sections. 



(h) Study stained sections taken at various levels along the antero- 

 posterior axis. Identify the structures previously observed, and com- 

 pare layer for layer the structures seen with the layers of the body 

 wall and gut wall of the frog. What kinds of cells do you find in each 

 of the layers observed? Can you observe any advance in cell special- 

 ization over the platyhelminths? How is the body cavity different 

 from that of the frog? What do your observations indicate regarding 

 the kinds of movements possible by the animal? Compare with the 

 planarian. 



OTHER NEMATODES 



Exercise 1. — Other Parasitic Species. 



(a) Observe demonstrations of hookworms, trichina worms, and 

 such other parasitic nematodes as are available. From textbook 

 figures and descriptions understand their life-cycles and the nature 

 of their parasitic existence. 



Exercise 2. — Free-living Nematodes. 



(b) Study nematodes obtained from marine or fresh-water infu- 

 sions or from moist soil, sandy beaches, and elsewhere. Note any 

 structural modifications related to their free-living habits, and con- 

 trast their structure with that of parasitic species. Homemade vinegar 

 often has numerous minute nematodes, the vinegar eels, Anguillula 

 aceti, present in it. Such cultures may be maintained for several years 

 by merely adding distilled water to maintain a constant volume of the 

 fluid. Rhabditis terricola, which lives in the soil and is often abun- 

 dant in decaying substances, is favorable for study. What structural 

 features of nematodes resemble the structures with which you are 

 familiar in other phyla? How do nematodes differ from these other 

 forms in regard to cell specialization and division of labor among cells, 

 tissues, and organs? 



