FLAT WORMS AND THREADWORMS 



307 



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 



A. Malacobdetta 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. Micrura leidy, (Verrill.) 



D. Cerebratulus lacteus (Ver- 

 rill). (From Pratt's "Manual" 

 by permission of A. C. McClurg 

 & Co.) 



. 195. 



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.) 



Cerebratulus ( ), and Micrura ( ), 



(Fig. 195), are the usual examples of marine Nemertinae. Other forms 

 are not common. Malacobdella ( ) is parasitic in 



some mollusks. 



The Nemertinae are considered the lowest form of animal life in 

 which the blood-vascular system appears. There is a definite mesoderm 

 and a nervous and an excretory system quite like those in flatworms. 

 but they all have a long proboscis just above the digestive tract which 

 lies within a sheath. This can be everted. The body is covered with 

 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) re- 

 sembling a helmet with cilia and a long tuft at the apex, is a distinguish- 

 ing feature of the development of Nemertinae. Ectodermal invagina- 

 tions surround the alimentary tract of the Pilidium. This invaginated 

 portion escapes from the larval form and becomes an adult. 



