CARNIVOROUS ANNELIDS AND SIPHON-WORMS 147 



of similar kind, as well as four eyes. The most extraordinary 

 arrangement, however, is to be found in the seizing organs. 

 Nothing is present at all comparable to the jaws or seizing 

 limbs of many backboned animals, nor are there jointed limbs, 

 which, as we have seen, are specialized for this purpose in both 

 air-breathing and gill -bearing arthropods. But the food -tube 

 begins with a muscular pharynx, which can be protruded so as 

 to bring into play a pair of formidable horny jaws which project 

 from its cavity. These having seized the prey, a reverse action 

 takes place, bringing it within the body. Small Crustacea, mol- 

 luscs, and sponges are the chief articles of diet. 



Comparable to the preceding in respect of its food, though with 

 weakly -developed jaws, is the Sea-Mouse (^Aphrodite aculeatd) 

 (fig. 406), which, however, is not a swimmer. Here the body is 



Fig. 406. — Sea-Mouse {Aphrodite aculeatd) 



short and broad, and there are very numerous elongated bristles 

 of beautifully iridescent appearance. The upper side is also 

 covered with a sort of loose skin composed of innumerable small 

 bristles matted together so as to enclose a space above the skin 

 proper. 



LEECHES (DiscoPHORA) 



Most of these are entirely devoid of the bristles so charac- 

 teristic of the preceding group, and there is a sucker at either 

 end of the body, the mouth-opening being in the middle of the 

 front one. Leeches are divided into those possessing jaws and 

 those devoid of these structures. 



Jawed Leeches. — The common Medicinal Leech {Hirudo 

 -medicinalis) (fig. 407) is a familiar freshwater type of the jaw- 

 bearing kind. It moves about either by swimming in an undu- 



