v PHYLUM PLATYHELMINTHES 145 



Bothriocephalus latus, a very large tape-worm without 

 hooks, is a common human parasite in eastern countries. 

 Its Cysticercus occurs in the pike and certain other fresh- 

 water fishes. It has not become endemic, or naturalized, 

 in the United States. 



4. THE NEMERTINEA 



The Nemerteans are non-parasitic, unsegmented worms, 

 most of which are marine, only a few forms living on land 

 or in fresh water. They are commonly looked upon as 

 nearly related to the Turbellaria, and were formerly in- 

 cluded in that class ; but they are in some respects higher 

 in organisation than the Turbellaria, and they exhibit cer- 

 tain special features distinguishing them from the rest of the 

 lower worms. 



The body (Figs. 79 and 80) is narrow and elongated, 

 cylindrical or depressed, unsegmented, and devoid of ap- 

 pendages. In length it varies, in different species, from a 

 few millimetres to as much as ten metres. The entire sur- 

 face is covered with vibratile cilia. 



The mouth (m) is at or near the anterior extremity on 

 the ventral aspect. Close to it above there is an opening 

 through which can be protruded a very long muscular organ, 

 the proboscis (fir), the possession of which is one of the 

 most characteristic features of this class of worms. The 

 proboscis is hollow : when it is extended to its utmost, a 

 part still remains which is not capable of being everted, and 

 at the junction between the eversible and non-eversible 

 parts, i.e., at the extremity of the organ when it is fully 

 protruded, there is in many of the Nemerteans a pointed or 

 serrated stylet (Fig. 80, st), which probably permits of the 

 proboscis being used as a weapon : when a stylet is absent, 



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