IV ANNELIDA 153 



genera which are aquatic, living in the mud at the bottom of fresh water 

 or, in a few cases, of the sea in close proximity to the coasts. 



The general characteristics of the group are well exemplified by 

 Lumbricus — the most important being the reduction in the number of 

 the chaetae, the disappearance of the parapodia, tentacles and cirri, and 

 the hermaphroditism with its correlated complexity of the reproductive 

 organs. It is also characteristic, as is usually the case with groups 

 originally marine which have taken to a fresh-water or terrestrial exist- 

 ence, that the active free-swimming larval stage has been eliminated 

 from the life-history, early development taking place within the cocoon. 



III. HiRUDINEA 



As an example of the group Hirudinea it will be convenient to examine 

 the ordinary leech used in medicine — belonging to the genus Hirudo. 

 The leech is a somewhat flattened worm measuring commonly about 

 3 to 5 inches in length although varying greatly according to the state 

 of extension or contraction of the body. 



The colour is green or brown with dark mottlings. The whole body 

 is marked off superficially into narrow rings or annuli, about 95 in all. 

 At the hind end of the body is a powerful round posterior sucker, while 

 at the front end the muscular lips of the wide mouth opening form an 

 anterior sucker. There are no traces of parapodia or chaetae. 



The alimentary canal of the leech is of special interest in its adapta- 

 tions to the peculiar feeding habits — the food being blood, which can be 

 obtained only at long and uncertain intervals. In the buccal cavity 

 arranged in radiating fashion are three small saws (Fig. 73 A,j), each 

 with a curved edge set with numerous small teeth of hard chitinous 

 material. The saws are provided with muscles by which they can be 

 rotated backwards and forwards so as to make a characteristic 3-rayed 

 cut in the skin of the animal attacked. The buccal cavity leads into 

 the pharynx (ph), ellipsoidal in shape and of furry appearance due to 

 the radiating muscles which pass from its surface to the body-wall. 

 These by their contraction cause the cavity of the pharynx to dilate 

 and in this way bring about a sucking action through the mouth. The 

 pharynx leads, with scarcely any intervening oesophagus, into an enormous 

 crop furnished on each side with ten or eleven blindly ending pockets 

 or caeca (c). These are least clearly marked in front, while on the other 

 hand the hindmost caecum on each side is very large. The crop serves 

 merely as a reservoir for the ingested blood, the actual digestion being 

 carried out in the intestine (int) — the narrow tubular portion of the 



