CHAPTER VII 



A Variety of Animal Groups 



N the animal kingdom are a number of small 

 groups whose members have charms for people with 

 the most observant eyes or a special curiosity and per- 

 sistence for seeking out animals of small size, few 

 species, or unobtrusive habits. 



All of these descriptions apply to members of the 

 phylum Mesozoa. They are minute ciliated worms 

 found living as parasites in the kidneys of squids and 

 octopuses, clinging to the walls of tubes while their 

 elongate bodies float freely in the urine. Mesozoans 

 parasitize many other invertebrates, finding homes 

 in the tissues and body cavities. Their habits remind 

 us of protozoans, but their bodies are multicellular — 

 more like the two-layered little planula larvae of 

 coelenterates. It is tempting to think of mesozoans as 

 being transitional between these groups, and this 

 temptation has led to the phylum name. Probably 

 they are degenerate in their simplicity, degraded by 

 parasitism, but they still appear to be the simplest of 

 multicellular animals — simpler than any flatworm 

 or coelenterate. 



Quite another kind of group is the Phylum Nema- 

 toda. enormously abundant, and with great numbers 

 of species, and boasting among its members some of 

 man"s most loyal companions, though they can 

 hardly be called friends. Nematodes are included 

 here only because they are thought to be related to 

 five of the small groups that follow them immediately 

 in this chapter. The six groups are often lumped to- 

 gether as six classes of a superphylum, Aschelmin- 

 thes, but the evidences for doing this, or for separat- 

 ing the six from certain other phyla in this chapter, 

 are too technical to be given here. Instead, each 

 group is awarded separate status as a phylum with 

 a distinct body plan. 



The Roundworms 



{Phylum Nematode!) 



The cost of minimizing one's enemies always runs 

 high, and we are now paying dearly for having so 

 long underestimated the prevalence and powers of 



roundworms. The big ascarids that live in the human 

 intestine were well known to the ancient Egyptians, 

 as one can hardly ignore a foot-long worm that slips 

 out with excrement when it dies, or one that may go 

 astray and suddenly emerge from a nostril. In our 

 own day ascarids are widespread in the world, in- 

 cluding Europe and the Americas, especially in 

 warm, moist areas. In the mountainous parts of the 

 southeastern United States, clay soil, a mild, rainy 

 climate, dense shade, and the habits of small chil- 

 dren, dogs, and pigs combine to spread and protect 

 Ascaris eggs in the dooryards where children play, 

 and from where they carry the eggs into their homes. 

 In hot, dry climates, especially from Arabia to India, 

 the 4-foot guinea worms that lie coiled under the 

 surface of the skin are even harder to overlook. Very 

 likely these are the same as the "fiery serpents'" that 

 plagued the Israelites in biblical times. 



Every species of vertebrate that is examined turns 

 out to harbor nematode parasites, and there are two 

 billion estimated cases of human infection, not much 

 less than the total number of people in the world. 

 Roundworms as huge as ascarids and guinea worms 

 are very exceptional, but parasitic forms are gener- 

 ally larger than the free-living ones. These last are 

 much more numerous and barely visible to the naked 

 eye. Magnified, they look like animated bits of fine 

 sewing thread, hence the phylum name, Nematoda, 

 which means "threadlike." Free-living roundworms 

 are inconceivably abundant in moist soils, present 

 even in deserts and on mountain tops, common in all 

 fresh waters, found in hot springs and arctic ice pools, 

 living in every sea from pole to pole. Yet small size 

 and transparency kept them unseen until after the 

 discovery of the microscope. They find their own 

 food, steadily devouring bacteria or small animals 

 and plants of soil and water. Their teeming numbers 

 and their versatility were noted by nineteenth-cen- 

 tury zoologists. 



In 1881 a German investigator, seeking to find 

 out why sugar beets, a mainstay of German agricul- 

 ture, seemed suddenly "to tire" of any soil in which 

 they had grown for many successive years, traced the 

 trouble to parasitic "eelworms." At first the study of 



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