234 



THE ANIMAL KINGDOM 



"Proboscis pore 

 "Vestibule. 



■Mouth 



-Proboscis 

 csLvity 



•Proboscis 

 sheath 



"Proboscis 



"Proboscis 

 retir'a.ctor 



-Anus 



AntGrior lacuncL 

 Brain 



■Mouth 



-Venfcro-lafce-ral nerve 

 "Late-ra-1 bloodvessel 

 'Digestive, tract 



Posterior lacuna. 



Anixs 



A B 



Figure 12.9. Diagrammatic views of nemertean structures. A, Lateral view of the 

 dieestive tract and the proboscis. B, Dorsal view of the digestive, circulatory and nervous 

 systems. 



The reproductive organs are simple, saclike structures scattered 

 along each side of the body. The sexes are usually separate and fer- 

 tilization is external. Eggs and sperm may be shed through short tubes 

 that develop from each gonad to the body surface, or they may merely 

 burst through the body wall. Most nemerteans have excellent powers of 

 regeneration, and some of them reproduce asexually by fragmenting into 

 a number of pieces, each of which becomes a whole worm. 



Questions 



1. Give examples for the six classes of the phylum Aschelminthes. 



2. Explain the illusion of rotation in rotifers. 



3. What do PInlodina and Turbatrix eat? 



4. W'hat is cell constancy and what is its apparent relation to regenerative capacity? 



5. How do roundworms move? 



