The Animal Kingdom - 645 



Fig. 32-21. A modern brachiopod {Tentaculata). A, diagram of internal struc- 

 ture; B, photo of three clustered individuals. (From General Zoology, by Miller 

 and Haub. Holt, Rinehart and Winston, Inc.; lower photo, copyright, General 

 Biological Supply House, Inc.) 



may be present; and typically there is a ring 

 of nervous tissue around the esophagus. 

 Generally the sexes are separate and the 

 males are distinctly smaller than the females. 



Some parasitic nematodes are very harm- 

 ful, economically and medically. The young 

 of the common garden nematode (Hetero- 

 dera marioni) penetrate and feed upon the 

 root tissues of quite a number of garden and 

 crop plants. These cause root swellings, or 

 galls, which may be very detrimental to the 

 yield. The "golden nematode" seems to have 

 entered this country quite recently, but al- 

 ready it is doing considerable damage to 

 several crops, especially potatoes, in locali- 

 ties where the infestation is heavy. 



Among medically encountered Nema- 

 thelminthes, hookworms, trichina, and filarial 

 worms are perhaps the most important. The 

 common American hookworm (Necator 

 americanus) stunts the growth and saps the 



strength of many unfortunate individuals in 

 some poorer localities where good sanitation 

 facilities are lacking and where people tend 

 to go barefooted. Eggs of the hookworm enter 

 the soil with the feces, and the active young 

 larvae penetrate the delicate skin of the vic- 

 tim, usually on the sides of the foot near the 

 toughened sole. In the body, the larvae mi- 

 grate to the lungs via the blood stream. Then 

 they crawl out of the lung, up through the 

 trachea, and into the digestive tract. The 

 adult worm, usually about 10 mm long, at- 

 taches itself to the intestinal wall. Here it 

 feeds upon blood, lymph, and bits of tissue, 

 obtained by means of the cutting action of 

 the minute teeth of the mouth and the suck- 

 ing action of the worm's muscular pharynx. 

 Infection by fifty to a hundred adult hook- 

 worms seriously impairs the health of the 

 host. Internal bleeding from the many small 

 intestinal wounds is aggravated by an anti- 



