310 



General Biology 



a. 



avicularia 



Fig. 199. 



Bugula avicularia, a Brjozoon. Av, 

 ; D, alimentary canal; F, funiculus; 

 Oes, oesophagus; Ovz, ovicells; R, retractor 

 muscle; Te, tentacular crown. (From Sedg- 

 wick, after V. Nordmann.) 



B. Phoronis 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.) 



ventral nerve-cord, while the ovaries 

 discharge the eggs into the body- 

 cavity. Then, too, the larvae of 

 Gordious 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 Acanthocephala 

 from the Nematoda and the Nematomorpha. There is an alternation 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. 

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

 thin-shelled and develop without fertilization (parthenogenetically). 

 The larger eggs produce only females while the small eggs reproduce 

 males. The winter eggs are fertilized, have thick shells, and all develop 

 into females. The eggs of the most mollusks pass through a larval stage 

 known as a trochophore ( ), which looks quite like 



the helmet-shaped larva described above. Now, Rotifers often resemble 

 these trochophores. Consequently, it is thought by some zoologists that 

 they must be closely related to the mollusks. Rotifers have a peculiar 

 ability to secrete about themselves in times of drought a gelatinous 

 envelope, which protects them for great lengths of time and thus prevents 

 them from perishing. 



The Bryozoa (Gr. Bryon, moss — zoon, animal) are moss-animals 



