FLAT- WORMS. 469 



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 

 enclose themselves in a capsule, and await the chance of being 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 amongst 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 forwards and outwards to supply the head and eyes, two stout 

 cords, one on each side of the body, run backwards to supply the tail. 



Suborder Rhabdoccela. 



This group is characterised 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- 

 coming its prey being somewhat Mesostomum tetragonum. 

 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 

 chance of escape. It is stated that this and other species ensnare their prey 

 by means of slimy threads. Eggs of two kinds are laid ; those found in summer 

 being soft-shelled, while those of winter are protected by a hard and thick 

 coat so as to be able to withstand the unfavourable conditions of this season. 

 An allied species (M. tetragonum) is shown in the accompanying illustration. 



