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 (pr), 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, 



