640 



APPENDIX B 



These smooth, glistening, slender worms occur in large numbers, both as regards 

 species and individuals, in soil and in fresh and salt water and as parasites in 

 plants and animals. The common vinegar eel (Fig. B.8) is an example of the free- 

 living roundworms; the horsehair "snake" is typical of others that spend part 

 of their lives as parasites and another part as free-living organisms. The 

 hookworms (Fig. B.9) and Ascaris are roundworms completely adapted to para- 

 sitic existence. 



Female 



Fig. B.9. A roundworm Necator americanus, the commoner of the two chief human hook- 

 worms in the United States. The adults live in the intestine, feeding on blood from the 

 intestinal walls. The eggs are passed with feces; they hatch into small larvae that can pene- 

 trate exposed skin, causing "ground itch." In the body they travel in the blood and lymph 

 vessels to the lungs, thence to the trachea and esophagus, and so to the intestine. This 

 figure shows the anatomy of the male and female worms, which is fairly typical of that of 

 nematodes in general, a, anus; ca, copulatory appendages; cs, copulatory setae; e, eggs in 

 oviduct; i, intestine; to, mouth; so sex openings; sv, seminal vesicle; t, testes. {Courtesy 

 General Biological Supply House, Inc.) 



Approximately 80,000 species of Nemathelminthes have been described. Partly 

 because of the extreme difficulty of recognizing species in this group, it is probable 

 that many more thousands remain to be discovered. 



Animals of Uncertain Relationship 



A number of groups of animals are considered together here because 

 their relationships to other animals and to each other, as well as their 

 position in an evolutionary sequence, are uncertain. All are triploblastic 

 and bilaterally symmetrical, and all show a degree of organization at 

 least as complex as that of the Platyhelminthes and Nemathelminthes. 



NEMERTINEA (nem' er tin' e a) . Worms, mostly marine, probably related to 

 the flatworms. Important characteristics: a protrusible proboscis that lies in an 

 anterior sheath (the latter considered by some the coelom) ; digestive tract tube- 

 like, with both mouth and anus; a blood- vascular system. They are the most 

 primitive animals with a circulatory system. 



ROTIFERA (ro tif er a). Rotifers or wheel animalcules. Minute but complex 

 animals, most of which live in fresh water, although some are marine and a few 

 parasitic. Movements of cilia at the anterior end suggest rotating wheels. Diges- 

 tive tract tubelike, with both mouth and anus; easily visible chitinous jaws 

 present within pharynx; 1,000 species. 



BRYOZOA (bri' o zo' a). Moss animals. Small, sessile, unsegmented, mostly 

 colonial animals, living in both fresh and salt water, though more abundant in 



