ix PHYLUM ANNULATA 203 



on the surface. They also feed on decaying leaves, and 

 sometimes on animal substances. Some of the fresh-water 

 Oligochaeta manufacture tubes of mud held together by a 

 tenaceous secretion from glands in the integument Some 

 of the Polychaeta move about freely : others burrow in sand 

 or even in rock, or in the shells of Molluscs ; some occupy 

 temporary tubes ; others inhabit permanent tubes sometimes 

 of parchment-like consistency, sometimes hardened by de- 

 position of grains of sand, small fragments of shells or other 

 foreign bodies, and sometimes of dense shelly calcareous 

 material. These tubes are usually firmly fixed to a rock 

 or sea-weed, or other foreign body. While the free-living 

 Polychaeta are carnivorous in their diet, those that inhabit 

 permanent tubes are vegetable feeders. 



A few Polychaeta are pelagic. The majority live among 

 sand, mud, rock or sea-weed in shallow water, or actually 

 between high- and low-water limits ; but they also occur at 

 all depths in the ocean. 



2. THE HIRUDINEA. 



A good example of the Hirudinea is the Medicinal Leech 

 (Hirudo), various species of which are to be found in ponds, 

 swamps, and slowly-flowing streams in all parts of the world. 



The Leech is a vermiform animal, some 6-10 cm. (2-3 

 inches) in length, but capable of contracting and elongating 

 itself so as to produce great alterations in form and propor- 

 tion. It moves by " looping " movements, and is also a good 

 swimmer. The body (Fig. in) is depressed or flattened 

 dorso-ventrally, the dorsal surface being convex, the ventral 

 flattened. The anterior end presents a ventrally directed 

 cup-like hollow, the anterior sucker (a. s.), in the middle of 

 which is a small aperture, the mouth (;;////.). The hinder 



