Survey of the Animal Kingdom 295 



Phylum 6. Nemathelminthes (nem a thel -min' thez) (Gr. nema, thread or round; 

 helmins, intestinal worm) 



General Characteristics 



These worms are elongated, slender, cylindroid, and with no internal or ex- 

 ternal segments (nonmetameric). They are bilaterally symmetrical. The animals 

 are triploblastic, having three primary germ layers: ectoderm, mesoderm, and 

 entoderm. The alimentary canal has a mouth at the anterior end and an anus on 

 the ventral side near the posterior end. There are no cilia on any part of the 

 body. The cavity between the internal organs and body wall is filled with loose, 

 mesenchymal tissue and probably is not a true coelom. The tubular sex organs 

 (gonads) are usually in separate individuals (diecious). Different species vary 

 in length from 0.01 to 1 meter. Nemathelminthes live in fresh and salt water, 

 damp earth, decaying matter, or parasitically in animals and plants. Certain 

 forms (Trichinella) live for a time embedded in the tissues, producing the disease 

 trichinosis in man, pigs, and rats. Such forms as the hookworm and Ascaris are 

 parasites in man and lower animals. Certain microscopic forms (vinegar eel) 

 live in vinegar. Number of species of Nemathelminthes, 8,000. 



Classification of the Phylum Nemathelminthes 



Class — Nematoda (nem a -to' da) (Gr. nematos, thread; eidos, form). — 

 The body is elongate, slender, cylindrical, and often tapered at the ends. There 

 are no segments but lateral lines are present. The digestive tract is straight and 

 nematodes have no proboscis. A resistant cuticle is shed (moulted) at intervals. 

 As a group they inhabit almost every possible habitat, many species living freely 

 in fresh water, salt water, or soil, while many other species parasitize other animals 

 and plants. (See table on p. 298.) 



Order 1 — Ascaroidea (as kar -oid' e a) (Gr. askaris, intestinal worm) . — 

 These organisms are free living in soils, fresh water, or salt water, or they may be 

 parasitic. The mouth usually has three lips. This order includes the majority 

 of the nematodes. 



Examples: Human ascaris (Ascaris lumhricoides) (Fig. 184), sheep ascaris 

 (Ascaris ovis), human pinworm (Enterohius vermicularis) , horse pinworm 

 (Oxyuris equi), a parasite of hundreds of plants (potato, tomato, lettuce, trees, 

 and weeds) (Heterodera [Caconema] radicicola), and "vinegar eel" (Turbatrix 

 [Anguillula] aceti) (Fig. 98), living in vinegar, stagnant water, and decaying 

 materials. 



Order 2 — Strongyloidea (stron jil -oid' e a) (Gr. strongylos, round). — 

 All of these worms are parasitic. They frequently enter the body through the 

 skin or in water. The esophagus is club shaped and is without a posterior bulb. 

 Males have caudal bursae; they are supported by rays. These parasites produce 

 many common diseases. 



Examples: American hookworm (Necator americanus) (Fig. 99), European 

 hookworm ( Ancylostoma duodenale), ground itchworm ( Ancylostoma braziliense) , 

 and bird gapeworm (Syngamus trachea). 



Order 3 — Filarioidea (fil ar -oid' e a) (L. filum, thread).— All of 

 these worms are parasitic, living in the blood, lymph, connective tissues, or mus- 

 cles of higher animals (vertebrates) ; they require an insect host for their trans- 



