x PHYLUM ANNULATA 477 



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 hardened by the 

 deposition of grains of sand or particles of broken shells or bits of 

 sea- weed ; sometimes (Fig. 390) 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, such as species of Polydora and Stylarioides, 

 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 Alciopidce and Tomopteris, as well 

 as, in a certain phase, the Nereidce and Syllidce, 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 Chaetopoda in the older 

 geological formations, though there are many burrows and tracks 

 which have been ascribed to members of that class. Tubes of 

 tubicolous Polychaeta have, however, been found in formations 

 dating from the Cambrian period onwards. Some tubes, not 

 distinguishable from those of the existing genus Spirorbis, are 

 found as far back as the Silurian ; and others, apparently closely 

 related to the living Serpula, as far back as the Carboniferous. 

 In addition there are a number of tubes of extinct forms ascribed 

 to the tubicolous Polychaeta. The horny jaws of various Polychseta 

 have been detected in strata from the Cambrian period onwards ; 

 and many tracks and burrows occurring in rocks of all ages are 

 ascribed, some with more, some with less certainty, to this group 

 of worms. No fossil remains of Oligochseta are known. 



APPENDIX I. TO THE CELETOPODA. 



CLASS MYZOSTOMIDA. 



The Myzostomida are a group of worms which appear to have 

 their nearest relatives in the Chsetopoda, though possessing certain 

 special features of their own. They are all external, or, in one case, 

 internal, parasites of various Crinoids both of the stalked and the 

 free varieties, or internal parasites of certain Starfishes. They are 

 disc-shaped animals (Fig. 391) (elongated in Stelechopus) devoid of 

 any trace of external segmentation. There are patches of cilia 

 here and there on both dorsal and ventral surfaces. At the sides 

 there are five pairs of parapodia (p), each with a chitinous hook and 



