IX 



PHYLUM ANNULATA 



199 



confined to the head end. All such marine worms belong 

 to the sub-class Polychaeta of the Chaetopoda. Though 

 many of them move about freely like Nereis, others live 

 permanently in tubes of a membranous or shelly material. 

 The tube-inhabiting Polychaeta (Figs. 106, 107) usually 

 present marked modifications of form in accordance with 



FIG. 107. Serpulre with their tubes. (After Quntrefages.) 



their mode of life. The branchiae, when present, are usually 

 confined to the head end, so that they can readily be thrust 

 out through the opening of the tube ; and the body is fre- 

 quently divisible into regions, owing to more or less marked 

 differences, in the development of the parapodia and in other 

 points, between the anterior part which can be thrust out 

 of the tube, and the posterior parts which habitually remain 



