THE WORMS 63 



of the stomach of the animal, and lives somewhere within its body. 

 The worm, however, does not usually grow to any great size or 

 develop any segments, but remains little more than a "head" with 

 hooks and suckers. If, however, the animal within which it is 

 living be devoured by another, the little head is set free and attaches 

 itself to the walls of the stomach of the new host, and then it grows 

 to an enormous length, forming hundreds of segments which 

 develop thousands of eggs, and are finally cast out one after another 

 through the alimentary canal of the host. 



The highest of the worms are called ^innelids, for their bodies 

 are ringed or divided by constrictions into a number of segments. 

 The head segment contains the mouth and is often provided Avith 

 feelers and eyes. Then follow a large number of body segments 

 quite similar each to each. These often bear flapper-like side 

 appendages which are both gills and feet. The posterior segment 

 usually has one or more pairs of feelers. The intestine runs straight 

 through the body from one end to the other, and the throat which 

 is often armed with teeth, can be turned inside out. There is a well 

 developed In-ain in the head segment and a chain of nerve fibres 

 extends down the lower side of the body, with a knot-like mass 

 of nerve cells in each segment. The eggs of many Annelids 

 are cast out into the water Avhere they divide into a number of 

 cells, and soon develop into little free-swimming larvre which 

 are apt to be tack-shaped, the body being elongated, and the head 

 broad and flat. There is a ring of waving cilia around the edge of 

 the broad head of the larva, and another at its posterior end. The 

 mouth is at the narrow edge of the head of the tack, and the intes- 

 tine bends at right angles and runs entirely through the body, open- 

 ing at the posterior end. There is usually a sense organ or brain 

 at the centre of the head of the tack, and often we find sensory bris- 

 tles or hairs at this place. Larvae of this sort are so common that a 

 special name, trochopliora, is given to them. Soon the flapper-like 

 legs develop along the sides, and the body becomes segmented and 

 gradually changes into the form of the adult worm. But in addi- 

 tion to this development from eggs many worms increase in even 

 more interesting ways. For example, in some forms called Syllidce, 

 we often find that eyes and feelers begin to develop at regular 

 intervals upon certain segments of the body of the worm, and then 



