56 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



the hinder part of the body is less elaborately de- 

 veloped than the anterior, which can be protruded 

 from the mouth of the leathery, sandy, or calcareous 

 tube. The lowest forms of the division have no setae 

 at all, and Polygordius, which may be taken as the re- 

 presentative of the Achseta, retains throughout life a 

 circlet of cilia at its anterior end. 



In another and lower division of the Annulata we 

 find that the setae are never more than eight at the 

 most in each bundle ; and such forms may be distin- 

 guished from the Polychseta, and known as the 

 Oligochaeta. Of these the best known form is 

 the common earthworm (Lumbricus), but all are 

 not, like it, terrestrial in habit ; JSTais and the 

 blood-worm (Tubifex) are inhabitants of fresh 

 water. 



Most appropriately, perhaps, associated with the 

 Annulata, but exhibiting a number of characters that 

 bring them into relation with the flat- worms, are the 

 leeches or H imdinea ; living on the blood of other 

 animals, as many of them do, they have the integu- 

 ment often developed at one or two points into 

 suckers, by means of which they attach themselves 

 to other animals, or to firm bodies, from which they 

 can extend themselves to seize or attach themselves 

 to their prey. 



Most closely allied to the Annulata, but best kept 

 in a separate division, are those marine worms of 

 which Sipunculus is the best known example ; for 

 these the old term of Gephyrea may be retained, 

 without prejudice to our views of the value of the 

 ideas which gave rise to the name. The body ex- 

 hibits no external segmentation ; they are remark- 

 able for possessing excretory organs of the kind 

 found in the Annulata, as well as those seen 

 in Rotifers ; in some cases the anus is not at 

 the hinder end of the boclv, but the intestine is so 



