CHAP. XV Backboneless Animals 231 



boscis" lying in a sheath along the back, a pair of enigmatical 

 ciliated pits on the head. The sexes are almost always separate. 

 Almost all Nemerteans are carnivorous, but two or three haunt other 

 animals in a manner which leads one to suspect some parasitism ; 

 thus Malacobdella lives within the shells of bivalve molluscs. We 

 find many of them under loose stones by the sea-shore ; one 

 beautiful form, Lineus marinus, sometimes measures over twelve 

 feet in length. Some, such as Cerebratulus, break very readily into 

 parts, even on slight provocation, and these parts are said to be 

 able to regrow the whole. To speculative zoologists, the Nemer- 

 teans are of great interest on account of the vertebrate affinities 

 which some of their structures suggest. Thus the sheath of the 

 "proboscis" has been compared with the vertebrate notochord 

 (the structure which precedes and is replaced by a backbone), and 

 the two ciliated head-pits with gill-slits. 



3rd Set of Worms. Nematheliuiiitlies or Bound- 

 Worms — Sth Class, Nematoda or Thread - Worms. — The 

 ' ' worms " of this class are usually long and cylindrical, and the 

 small ones are like threads. The skin is firm, the body is 

 muscular ; in most a simple food-canal extends from end to end of 

 the body-cavity now for the first time distinct ; the sexes are 

 separate. Many of the Nematodes live in damp earth and in 

 rottenness ; many are, during part of their life, parasitic in animals 

 or plants. We have already noticed how long some of them — 

 " paste - eels," " vinegar - eels," etc. — may lie in a dried -up state 

 without dying. The life-histories are often full of vicissitudes ; thus 

 the mildew-worm {Tylmchus tritici) passes from the earth into the 

 ears of wheat, and many others make a similar change ; the female 

 of Sphcsrularia bomhi migrates from damp earth into humble-bees, 

 and there produces young which find their way out ; others, e.g. 

 some of the thread-worms found in man ( Oxyuris, Trichocephalus), 

 pass from water into their hosts ; others are transferred from one 

 host to another ; as in the case of the Trichina with which pigs 

 are infected by eating rats, and men infected by eating diseased 

 pigs, or the small Filaria sanguinis hominis, sometimes found in 

 the blood of man, which seems to pass its youth in a mosquito. 

 Somewhat different from the other Nematodes are those of which 

 the horse-hair worm Gordius is a type. They are sometimes found 

 inside animals (water-insects, molluscs, fish, frog, etc.), at other 

 times they appear in great numbers in the pools, being, according 

 to popular superstition, vivified horse-hairs. 



6th Class, Acanthocephala. — Including one peculiar genus of 

 parasites (Echinorhynchus'). 



4th Series of Worms. The Annelids or Ringed Worms 



— 7th Class, Chsetopoda or Bristle - footed "worms." — In the 



