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 Rhabdocoela. 
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 uj)on 
fresh-water worms, insects, and 
crustaceans, its way of over¬ 
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 
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. 
Mesostomum tetrarjon urn. 
