476 



ZOOLOGY 



SECT. 



in a string before undergoing separation. These sexual zooids 

 become developed into mature males or females, which may be 

 remarkably unlike the parent form in the shape of the parapodia, 



the character of the 

 setae, and other 

 points ; and in 

 some instances the 

 two sexes not only 

 differ from the 

 asexual parent form 

 but also from one 

 another, so that the 

 three forms, before 

 their relationship 

 was known, were 

 set down as repre- 

 senting three dis- 



FIG. 38i. Portion of Syllis ramosa. (From the Cambridge + : nn 4. , 



Natural History, after Mclntosh.) 



Syllis ramosa 



(Fig. 389), which occurs in the interior of certain deep-sea sponges, 

 is exceptional among the Chaetopoda in giving rise by lateral 

 branching to a colony from which 

 sexual zooids afterwards become 

 separated off. 



Modes of Life, etc. Very few 

 Chsetopoda are true parasites ; but a 

 considerable number are to be set 

 down as commensals, habitually asso- 

 ciating with another animal for the 

 sake of food and shelter. The Earth- 

 worms 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 Oligochaeta 

 (Tubificidce) manufacture tubes of 

 mud held together by a tenacious 

 secretion from the epidermal unicel- 

 lular glands. Some Of the Phanero- FIG. 39 J.Serpulae with their tubes. 



cephala form temporary tubes of a 



gelatinous character, or more permanent parchment-like tubes some- 

 times strengthened by means of agglutinated sand-grains. But the 

 majority of the Phanerocephala, which for the most part prey on 

 other small animals, are not confined to tubes, but move about 



