i8o UNSEGMENTED " WORMS." 



Development. 



(i.) In Cerebratulus, etc., the larva is adapted for pelagic 

 life, and is known as the Pilidium. " In external shape it 

 resembles a helmet with spike and ear lobes, the spike being 

 a strong and long flagellum or a tuft of long cilia, the ear 

 lobes, lateral ciliated appendages" (Hubrecht). (2.) In 

 Linens there is a sedentary larva, which has been interpreted 

 as a reduced Pilidium, and is known as the "larva of Desor." 

 (3.) In Hoplonemertea, the development is direct without 

 metamorphosis. 



Habits. 



Most Nemertines are marine, creeping about in the mud, 

 under stones, among seaweed, and the like ; many are able 

 to swim; Pelagonemertes is pelagic; a few live in fresh water; 

 Malacobdella lives in the mantle cavity of marine bivalves, 

 and two others occur on crabs. Most seem to be carni- 

 vorous, eating other "worms." Many break readily into 

 pieces when stimulated, and the Schizonemertea are able to 

 regenerate what they lose in this way. 



Classification (after Hubrecht) : 



1. Palceonemertea : No deep head fissure ; no stylet ; mouth behind 



brain. 



e.g., Carinella, Cephalothrix, Carinoma, Polia. 



2. Schizoneuierlea : A deep head fissure with a ciliated duct to the 



brain ; lateral nerves between the longitudinal and inner 

 circular muscles ; mouth behind brain. 

 e.g.) Lineus, Cerebratulus^ Langia. 



3. Hoplonemertea : No deep head fissures; lateral nerves inside the 



muscles ; stylet present ; mouth generally in front of brain. 

 e.g.) Amphiporus, Nemertes, Drepanophorus, Malacobdella^ 

 The last has no head fissures nor spines on the pro- 

 boscis, but bears a posterior sucker. 



Relationships. Some of the characteristics of the Nemerteans are 

 hinted at among the Turbellarians. Professor Hubrecht has maintained 

 that Nemerteans exhibit affinities with Vertebrates. (See Chapter XX.) 



Class NEMATODA. Threadworms, Hairworms, c. 



The Nematodes are unsegmented, more or less thread-like 

 "worms" some of which are free living and others parasitic. 

 The body is covered by a cuticle, often thick. Ciliated epithe- 



