488 



ZOOLOGY 



Modes of Life, etc. Very few Chaetopoda are true parasites ; 

 but a considerable number are to be set down as commensals, 

 habitually associating 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 

 subsequently throwing it out in the shape of castings on the 

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

 animal substances. Some of the fresh-water Oligocha?ta (Tubificidrr) 

 manufacture tubes of mud held together by a tenacious secretion 

 from the epidermal unicellular glands. Some of the Phanerocephala 

 form temporary tubes of a gelatinous character, or more permanent 



parchment-like tubes sometimes 

 strengthened by means of agglu- 

 tinated sand-grains. But the ma- 

 jority of the Phanerocephala, which 

 for the most part prey on other 

 small animals, are not confined t<> 

 tubes, but move about freely. Some 

 burrow in sand ; others even in 

 harder substances, such as the shells 

 of Mollusca, or in limestone, shale, 

 or sandstone. Many Cryptocephala 

 secrete tubes the substance of which 

 is derived from the epidermal 

 glands. These tubes are sometimes 

 membranous or parchment - like, 

 sometimes membranous but har- 

 dened by the deposition of grains 

 of sand or particles of broken shells 

 or bits of sea-weed ; sometimes 

 (Fig. 383) they are of a hard, shelly, 

 calcareous character, sometimes 

 composed entirely of foreign par- 

 ticles cemented together ; very fre- 

 quently they are permanently fixed 



to foreign objects. Some, such as species of Polydora and Sty- 

 larioides, near relatives of which construct tubes, excavate galleries 

 in rock or coral or in the shells of various Mollusca. 



A few Polychseta, such as the Alciopidcu and Tomvpter'is. as well 

 as, in a certain phase, the Ncreidw and Syllidcc, are pelagic ; but 

 the majority live on the sea-bottom. They occur in the greatest 

 abundance near the shore ; but are also found at all depths in the 

 ocean, the tube-dwelling forms being more abundant than the free 

 forms in the deeper zones. 



Owing to the soft character of most of their parts, there are 

 comparatively few actual remains of Chsetopoda in the older 

 geological formations, though there are many burrows and tracks 



Fio. 383. Serpulae with their tubes. 

 (After Quatvefages.) 



