THIE WORMS 638 
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 Annelids, for their bodies 
are ringed or divided by constrictions into a number of segments. 
The head segment contains the mouth and is often provided with 
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 brain 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 where they divide into a number of 
cells, and soon develop into little free-swimming larvee 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. Larvee of this sort are so common that a 
special name, trochophora, 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. [or example, in some forms called Syllide, 
we often find that eyes and feelers begin to develop at regular 
intervals upon certain segments of the body of the worm, and then 
