202 MANUAL OF ZOOLOGY sect. 



nourished by swallowing the albuminous fluid contained in 

 the cocoon. 



The earthworms, together with a number of allied fresh- 

 water forms, constitute the sub-class Oligochseta of the 

 Chaetopoda. As a group they are distinguished from the 

 Polychaeta by the sexes being united in the same individual, 

 with the ovaries and testes compact and few in number, by 

 the absence of parapodia and cirri, the non-development of 

 a distinct head-region, and the absence of a free larval stage. 



Very few Chaetopoda are true parasites, but a considerable 

 number are to be set down as commensals, habitually associ- 

 ating with another animal for the sake of food and shelter. 

 The earthworms burrow in soil containing decaying vegetable 

 matter, passing the mould through their intestine and subse- 

 quently throwing it off in the shape of " castings " on the 

 surface. They also feed on decaying leaves, and sometimes 

 on animal substances. Some of the fresh-water Oligochseta 

 manufacture tubes of mud held together by a tenacious 

 secretion from glands in the integument. Some of the 

 Polychaeta move about freely or burrow in sand, or even in 

 rock or in the shells of molluscs ; some occupy temporary 

 tubes ; others inhabit permanent tubes sometimes of parch- 

 ment-like consistency, sometimes hardened by deposition of 

 grains of sand, small fragments of shell or other foreign 

 bodies, sometimes of dense, shelly, calcareous material. 

 These tubes are usually firmly fixed to a rock or a seaweed 

 or other foreign body. While the free-living Polychseta are 

 carnivorous in their diet, those that inhabit permanent tubes 

 are vegetable feeders. 



A few Polychseta are pelagic. The majority live among 

 sand, mud, rock, or seaweed in shallow water, or actually 

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

 all depths in the ocean. 



