THE CLASSIFICATION OF ANIMALS 41 



how the scourge was to be dealt with. Many thread-worms 

 infest the flesh of the pig, and one microscopic form escapes 

 when this flesh is eaten and takes up its abode in the intestine 

 of the flesh-consumer. Thence it burrows through the walls 

 of his stomach into his flesh, causing in its passage the severe 

 and often fatal disease called Trichinosis, which has wrought 

 great havoc in Germany, where pig-flesh is often eaten in 

 an imperfectly cooked condition. 



Still another phylum of " worms " exists, which are termed 

 Annelida or Ringed- worms. This group 

 includes our familiar earthworms, and the 

 innumerable forms of marine worms, and, 

 last but not least, the leeches. The lead- 

 ing feature in this group is the repetition 

 of similar parts or organs in a line one 

 behind the other. This kind of repetition 

 plays a great part in the architecture of 

 the bodies of the higher animals, and is 

 known as metamerism. Thus, if we 

 look at the body of the earthworm, we 

 can see that it is divided by a series of 

 constrictions into a set of rings. Each 

 ring is termed a " somite," and in each 

 we find embedded eight bristles. Each 

 somite contains a pair of ganglia of the FIG. 15. Upper view 

 central nervous system. The nervous & 

 system is indeed one of the most charac- repeated bristles; 

 teristic features of the Annelida. It con- som, a somite, 

 sists of a pair of ganglia, closely con- 

 nective, lying above the digestive tube in front and termed the 

 brain ; and a series of pairs of closely connected ganglia lying 

 beneath the gut one pair, as we have already said, being 

 situated in each somite. These lower ganglia are connected with 

 one another by longitudinal bands of nervous matter or commis- 

 sures. The whole knotted chain is termed the ventral nerve- 

 cord, and this cord is connected with the brain by a " nerve- 

 collar " which embraces the digestive tube in front. The under 

 side of an animal is called " ventral " (Lat. venter, the belly) ; 

 the upper side is termed " dorsal " (Lat. dorsum, the back). In 

 the Annelida we meet for the first time with a secondary body- 



