4 6o 



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 



