VERMES. tSi 



sac-like body, which develops within itself from asex- 

 ual germ-cells small creatures called Redice : these ia 

 turn similarly develop others with suckers and a tail, 

 called Gercarioe. These get out of the snail, and swim 

 about in water. Then they encyst, sticking to the 

 grass, and get eaten by sheep, within which they 

 assume the sexual worm form, the fertilized eggs of 

 which again go through the same cycle. The Gestoda 

 include the Tape-worm, Tcenia, which presents a 

 somewhat similar life-history, the young tapeworms 

 being developed asexually out of a parent cyst con- 

 tained in one animal, and attaining their full length 

 and maturity when they get into another animal which 

 has eaten the first one. The adult animal, owing to 

 its parasitic existence, has assumed a special type of 

 structure, being possessed of no alimentary canal. 



The Nemertine worms, or Bhynchocoela, possess a 

 larva called a pilidium, which is in some respects 'like 

 that of some Echinoderms. These Nemertines have 

 a proboscis (Gk. 'jrpo^oaici<i, elephant's trunk; snout), 

 which can be thrust out or drawn in, and is used for 

 attacking their prey. It is provided with a sheath; 

 a theory has been suggested that this sheath is the 

 original form of the notochord of vertebrates (see pp. 

 76 and 78), and that the ancestors of vertebrates were 

 accordingly a group allied to the Nemertines. 



The Nematode Worms, class Nemathelminthes, in- 

 clude parasitic worms, such as thread-worms and the 

 Trichina, which causes an acute and often fatal 

 disease. 



