308 



General Biology 



Fig. 194. 



A. A Mesozoon, 

 Dicyema Paradoxum. 



(From Parker and 



Haswell, after 



Kolliker.) 



B. A Mesozoon, 

 Rhopalura giardii, 



male, 



(From Sedgwick, 



After v. Beneden.) 



C D Fig. 



A. Malacobdella grossa (Ver- 

 rill), entire worm. 1, proboscis; 

 2, mouth; 3, intestine; 4, 

 sucker. 



B. Section through forward 

 end. 1, mouth; 2, proboscis; 3, 

 proboscis sheath. 



C. M (crura leidy, (Verrill.) 



D. Cerebratulus lacteus (Ver- 

 rill). (From Pratt's "Manual" 

 by permission of A. C. McClurg 

 & Co.) 



195. E 



E. Cerebratulus fuscus, a Ne- 

 mertine. 1, cephalic slits; 2, 

 opening leading into retracted 

 proboscis; 3, dorsal commis- 

 sure of nervous system; 4, 

 ventral commissure; 5, brain; 6, 

 posterior lobe of brain; 7, 

 mouth; 8, proboscis; 9, lateral 

 vessel; 10, proboscis; 11, 

 pouches of alimentary canal; 12, 

 stomach. (From Shipley and 

 MacBride, after Burger.) 



cilia. These animals feed on other animals both dead and alive. They 

 usually live in burrows of mud and sand, though Cerebratulus is free- 

 swimming. 



A peculiar larval stage known as the Pilidium (Fig. 198 D), 

 resembling a helmet with cilia and a long tuft at the apex, is a dis- 

 tinguishing feature of the development of Nemertinae. Ectodermal 

 invaginations surround the alimentary tract of the Pilidium. This 

 invaginated portion escapes from the larval form and becomes an adult. 



The Nematomorpha (Gr. nema, thread — morphe, form), is made up 

 of the single family Gordiidae (Fig. 196). These are the common horse- 

 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 



