NEMERTINA OR NEMERTEANS. 159 



The Cestodes are closely connected with Trematodes 

 by such forms as Amphilina, Caryophyllaus, Archigetes. 

 Zoologically, they are interesting on account of their life- 

 histories, the degeneration associated with their parasitism, 

 the prevalence of self-impregnation, and the complexity of 

 the reproductive organs. Practically, they are of importance 

 as parasites of man and domestic animals. The medical 

 student should consult Leuckart's great work, The Parasites 

 of Man, part of which has been translated by W. E. Hoyle 

 (Edin. 1886), 



SECOND SERIES OF "WORMS." 



Class Nemertina. Nemerteans. 



This class of worm-like animals is often included among 

 the Plathelminthes, along with Turbellarians, Trematodes, 

 and Cestodes. Like the Plathelminthes, Nemerteans are 

 bilaterally symmetrical, unsegmented, and somewhat flat, but 

 their deeper characteristics warrant us in giving them a 

 separate position. 



General Characteristics. 



The Nemerteans, or ribbon-worms, are almost exclusively 

 marine, and are common under stones at low tide. They are 

 carnivorous in habit, and often very long and elastic, notably 

 in the caseof Zz«««j,of whose uncoiled length I have measured 

 twelve feet. When captured or injured they readily break 

 into parts, which can sometimes regenerate the whole body. 



The animal tends to grow lank; the body is unsegmented, 

 but there is sometimes an internal repetition of parts ; the 

 ectoderm is ciliated. A few swim, most creep slowly, partly 

 by the movements of the cilia, partly by the muscular 

 contractions of the body. 



A well-developed straight or slightly pouched food-canal 

 extends through the body, and there is virtually no body-cavity. 



Above the food-canal, and independent of it, there lies 

 within a definite sheath an eversible poisonous, and often 

 spine-bearing, proboscis. It is surrounded by a nerve-ring 

 from the brain, is probably both offensive and tactile, is 

 sometimes nearly as long as the worm, and is thrown out at 

 an anterior aperture usually situated above the mouth. 



