3442 THE WORM-LIKE ANIMALS 



received the name Cercaria. It escapes from the parent Redia by an aperture 

 situated near the front end of its body; and presents a considerable resemblance to 

 a tadpole, consisting of a long vibratile tail, and a wide heartshaped body with a 

 forked intestine, two suckers, and a gland on each side of the intestine. These 

 larvae make their way out of the snail, but instead of seeking a new host swim 

 about for a time and ultimately settle upon some water plant, or a blade of grass in 

 a meadow. Here they inclose themselves in a capsule, and await the chance of be- 

 ing devoured along with the grass by a sheep. From the stomach of the sheep they 

 make their way into the bile ducts, and there develop into the mature stage. 



TURBELLARIAN WORMS Order TURBELLARIA 



Nearly all the worms of this group lead a free life, parasitism among them 

 being the exception. They are found either on the land or in fresh or salt water, 

 and have received the name Turbellaria or whirl worms, from the whirling eddies 

 of water caused by the lashing of the cilia with which their unsegmented and 

 flattened bodies are covered. In shape they vary considerably, being either short 

 and oval, or long and worm-like. The alimentary canal is almost always well 

 developed, having a distinct mouth, but never a posterior outlet. There are no 

 special respiratory or circulatory organs, the function of breathing being performed 

 by the entire surface of the skin. The nervous system consists of a large double- 

 lobed brain ganglion, from which, in addition to some branches passing forward 

 and outward to supply the head and eyes, two stout cords, one on each side of the 

 body, run backward to supply the tail. 



SUBORDER Rhabdoccela 



This group is characterized by the straight and unbranched intestine. 

 A fairly well-known member of the suborder is Mesostomum ehrenbergi, a species, 

 about half an inch long, found during spring and summer in ponds and 

 streams. The generic name refers to the fact that the mouth, with its muscular 

 gullet, is situated in the middle of the lower surface of the body. Although as 

 transparent as glass and extremely fragile, it is a swift swimmer, moving quietly 



through the water either by 

 means of its cilia, or by waving 

 the edges of the exceedingly 

 flexible body. It feeds upon 

 fresh-water worms, insects, and 

 crustaceans, its way of over- 

 Mesostomum tetragonum. -coming its prey being somewhat 



curious. The worm converts its 



body into a cup-shaped hollow, at the bottom of which the mouth is situated, by 

 bringing the two ends close together, and turning over the flexible edges in the 

 same direction. In this manner it envelops its prey, and so deprives it of all 



