682 ' VERMES CLASS ROTIFER A. 



alimentary canal of the animal from which they set out. The migrations 

 of other species is substantially similar to that of E. gigas. For instance, 

 E. polymorphiis lives as a mature animal in the intestine of ducks or other 

 water-fowl, while in the immature stage it occurs in the little fresh-water 

 crustacean (Gammarus pulex). This amphipod (G. pulex) is also, in conjunc- 

 tion with the fresh-water wood-louse (Asellus aquaticus), the intermediate 

 host of two other species, E. anyustatus and E. proteus, which, when 

 mature, infest the alimentary canal of fresh-water fish. Instances of the 

 occurrence of worms of this group in mankind have been recorded, but, 

 happily, such cases are rare. 



Although, probably, more nearly related to the Thread-worms than to any 

 other group in the animal kingdom, the Arrow-worm (Sagitta) presents so 

 many peculiarities of organisation that its true systematic position is still a 

 matter of doubt, and it forms the sole type of an order known as Chsetognatha. 

 This name, meaning "bristle- jawed," is given to it in allusion to the fact 

 that the head is armed with a number of strong, movable, horny spikes which 

 act as jaws. The body is long, slender, and unsegmented, with a rounded, 

 blunt head, bearing eyes, and a narrow-pointed tail ; but the tip of the tail 

 and the sides of the body in its hinder half are furnished with horizontally- 

 projecting fins (Fig. 9, b). 



The Arrow-worms, so called from their arrow-like shape, are not parasitic 

 like the Thread-worms, but are to be found swimming in numbers on the 

 surface of the ocean in all parts of the world. 



THE WHEEL-ANIMALCULES. 



CLASS ROTIFERA. 



The Rotifera, or " Wheel-animalcules," as they were formerly called, on 

 account of the appearance in the water of a rapidly rotating wheel, to which 

 the movement of the circlet of hairs on their heads gives rise, are all 

 small, aquatic animals, varying in length from \ to 5^ of an inch, and found 

 universally in marine and fresh water, but most abundantly in stagnant 

 pools. 



The front end of the body is usually broader than the opposite extremity, 

 and is shaped like a disc, the trochal disc, which is 

 fringed with long vibratile hairs or cilia. By means 

 of the currents of water set up by the movement of 

 these cilia, particles of food are swept into the mouth, 

 a wide aperture placed in the middle or at the 

 sides of the trochal disc. The mouth passes into 

 the stomach, which is supplied with a complicated 

 armature of teeth, and by means of these the food is 

 crushed and divided. In the females the alimentary 

 canal passes right through the body, but in the males, 

 Fig. 13. WHEEL-ANIMAL- strange to say, it has ceased to be functional, and is 

 CULES. reduced to a solid chord-like organ. Apart from the 



a, floscularia coronetta. cilia of fche trocna l disc, the integument of the body 

 6, Pedalion mirum. ig g^^h, an d the only signs of segmentation that it 



C> ^Stefnudsonf ^ Presents are a few weak wrinkles. The tail-end, 

 or the foot, as it is called, varies considerably 

 in structure according to the habits of the species. In some forms that are 



