FLAT WORMS AND THREADWORMS 



309 



A. 



Av, 



hair snakes. Various authors classify them under the order of Nema- 

 toda, while others classify them under the Phylum Nemathelminthes. 

 There are two genera : Gordius, which lives in fresh water, and Necto- 

 nema, a marine form. The internal anatomy is somewhat different from 

 the Nematodes, as there is a distinct epithelium lining the body cavity 

 and no lateral lines. There is also a pharyngeal nerve-ring, and a single 



ventral nerve-cord, while the ovaries 

 discharge the eggs into the body- 

 cavity. Then, too, the larvae of 

 Gordius usually enter immature 

 stages of aquatic insects. These in- 

 sect larval-forms are then devoured 

 by other animals, and it is in the 

 intestines of the host where they de- 

 velop until they finally escape into 

 the water. 



The Acanthocephala ( G r. 

 akantha, spine kephale, head) are 

 the parasitic worms already men- 

 tioned above (Fig. 193), which may 

 infect man. They fasten themselves 

 to the intestinal wall of their host 

 by means of a protrusible proboscis 

 covered with hooks. In fact, it is 

 the presence of the proboscis and a 

 reproductive system as well as the 



absence of an alimentary system which distinguishes the Acantho- 

 cephala from the Nematoda and the Nematomorpha. There is an alter- 

 nation of hosts during the developmental stages. 



The Chaetognatha (Gr. chaite, horse-hair-gnathos, jaw) are marine 

 forms swimming about near the surface of the water. The arrow-worm 

 (Fig. 197) is the classic example. This is a member of the genus Sagitta. 

 The Chaetognatha are quite often included under the Phylum Nemathel- 

 minthes. 



The Rotifera or Rotatoria (Fig. 198), (Lat. rota, wheel-fero, I 

 carry), are usually called the wheel-animalcules. They are very small 

 and were formerly thought to belong to the Infusoria. Most of them 

 live in fresh water. A few are parasitic. The sexes are separate. There 

 are summer and winter eggs produced by the female. The former are 

 thin-shelled and develop without fertilization (parthenogenetically). 

 The larger eggs produce only females and the small males. The winter 

 eggs are fertilized, have thick shells, and all develop into females. The 

 eggs of most mollusks pass through a larval stage known as a trocho- 

 phore ( ). which looks quite like the helmet-shaped 



larva .described above. Now, Rotifers often resemble these trochophores. 



Fig. 199. 



Bugula avicularia, a. Bryozoon. 

 avicularia ; D, alimentary canal ; F, funiculus ; 

 Oes, oesophagus ; Ovz, ovicells ; R, retractor 

 muscle; Te, tentacular crown. (From Sedg- 

 vdck, after V. Nordmann.) 



B. Phorords architecta. Young individ- 

 ual with about 30 tentacles. 1, epistome ; 2, 

 lophophore ; 3, digestive tract. (From Pratt's 

 'Manual" by permission of A. C. McClurg 

 & Co.) 



