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 1 

 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 Oligochseta (Tubiji- 

 < it /re} manufacture tubes of mud 

 held together by a tenaceous secre- 

 tion from the epidermal unicel- 

 lular glands. Some of the Errant 

 Polycha?ta 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) they 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 Stylarioidcs, 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 Polychasta, such as the Alciopidcc and Tomopteris, as well 

 as, in a certain phase, the Ncrcida- and Syllidcv, 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. 3-iO. Serpulse with their tubes. 

 (After Quatrefages.) 



