460 
WORMS. 
mature state this worm, which spends this stage of its existence in the human 
intestines, may reach a length of 5 or 6 yards. Its head is relatively minute, 
being only about equal to that of an average pin in size, and furnished with 
four suckers, by means of which the creature adheres firmly to the walls of the 
intestine. The head is followed by a narrow piece called the neck, which gradually 
passes posteriorly into the trunk. It is not jointed, but where it merges with the 
a nemertine, Pterosoma -planum (enlarged). 
trunk it becomes marked by shallow grooves, growing deeper and deeper as they 
recede from the head, until ultimately they divide up the body into a chain of 
flattened, square or oblong segments, of which there may be many hundreds. 
Each segment is called a proglottis, the whole series being termed 'proglottides. 
The muscular system is fairly well developed, and consists of fibres running 
lengthwise throughout the segments and across from side to side, and of others 
passing from the upper to the lower walls. By means of these muscles the worm 
is able to shift at will its point of attachment to the gut, and to lengthen or 
