PHYLUM ANNULATA 



447 



lateral branching to a colony, from which sexual zooids afterwards 

 become separated off. 



Mode of Life, etc. Very few Chsetopoda 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 off in the shape of castings on the 

 surface. They also feed on de- 

 caying leaves, and sometimes on 

 animal substances. Some of the 

 fresh-water Oligochaeta (Tubiji- 

 cidce) manufacture tubes of mud 

 held together by a tenaceous secre- 

 tion from the epidermal unicel- 

 lular glands. Some of the Errant 

 Polychseta form temporary tubes 

 of a gelatinous character, or more 

 permanent parchment-like tubes 

 sometimes strengthened by means 

 of agglutinated sand-grains. But 

 the majority of the Errantia, which 

 live for the most part on other 

 small animals, are not confined to 

 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. The Tubicola 

 secrete tubes the substance of 

 which is derived from the epider- 

 mal glands. These tubes are some- 

 times membranaceous or parchment-like, sometimes membran- 

 aceous but hardened by the deposition of grains of sand or particles 

 of broken shells or bits of sea-weed ; sometimes (Fig. 349) thev are 

 of a hard, shelly, calcareous character, sometimes composed entirely 

 of foreign particles cemented together ; very frequently they are 

 permanently fixed to foreign objects. Some Tubicola, such as 

 species of Polydora and Stylarioides, near relatives of which con- 

 struct tubes, excavate galleries in rock or coral, or in the shells of 

 various Mollusca. The Tubicola are vegetable-feeders. 



A few Polycha3ta, such as the Alciopidce and Tomopteris, as well 

 as, in a certain phase, the Nereidw and Syllidce, are pelagic, but the 

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

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

 Tubicola being more abundant than the Errantia in the deeper 

 zones. 



FIG. 349. Serpulse with their tubes. 

 (After Quatrefages,) 





