THE BAKTHWORM. 149 



can have, although no one worm can adequately repre- 

 sent the whole branch. In fact, it would perhaps be 

 better to break up the branch of Vermes into several 

 branches, because of the great difference between 

 members of the group. It seems as if the men who 

 defined the limits of the branches of the animal king- 

 dom had picked out the animals that clearly belonged 

 to other groups, and then said, "We'll call all the rest 

 of the animals worms." There are commonly reck- 

 oned about ten classes of worms, all very different 

 from each other. 



Some worms are round, as the earthworm is, and 

 some are flat, like the leech. Some have suckers 

 at one end or at both ends. Some have the mouth at 

 one end of the body, and some have it in the middle. 

 Some have septa between the segments, and some 

 have no septa. Some lay eggs, and some are vivip- 

 arous ; that is, the young are born alive. This is 

 true of the vinegar worm, and of the trichina, a para- 

 site which infests pork. Some worms (Brachiopods), 

 have a shell, like a clam shell, and others have no 

 shell. In fact, moUusks themselves may be consid- 

 ered as only a highly specialized group of worms. It 

 is almost impossible, from a study of a sufficiently 

 varied number of specimens, to make a table of resem- 

 blances from which to derive a definition of worms which 

 will discriminate this branch from all other branches. 



Most of the worms are aquatic, and the greater num- 

 ber live in the sea. This fact we would expect to be 

 true from their lack of breathing organs and consequent 

 necessity of keeping the body moist. Many worms are 

 parasitic, as the trichina, already mentioned, and the 

 tapeworm. The Gordius,or hair worm, which is quite 

 common, is parasitic in one stage of its existence. 

 The very common belief that the hair worm is a trans- 

 formed horse hair, is not only unfounded, but fails to 

 recognize that a Gordius has various complex and 



