x PHYLUM ANNULATA 480 



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

 tubiculous 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 Polycha^ta 

 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 TO THE CH^ETOPODA. 

 CLASS MYZOSTOMIDA. 



The Myzostomida are a group of worms which appear to have 

 their nearest relatives in the Chaetopoda, though possessing certain 

 special features of their own. They are all external parasites of 

 various Crinoids both of the stalked and the free varieties, or 

 internal parasites of certain Starfishes. They are disc-shaped 

 animals (Fig. 384) devoid of any trace of external segmenta- 

 tion. There are patches of cilia here and there on both dorsal 

 and ventral surfaces. At the sides there are five pairs of para- 

 podia (p), each with a chitinous hook and a supporting rod; in 

 the intervals between these there are in Myzostoma four pairs of 

 small " suckers " ; and round the margin are a series of ten or 

 more pairs of cirri provided terminally with motionless sensory 

 cilia, and with a ventral groove lined by adhesive cells. The 

 mouth, usually situated at the anterior extremity, leads into a 

 muscular pharynx (Fig. 385, pJi.) capable of being protruded as a 

 proboscis ; from this a narrow oesophagus leads to the stomach, 

 which gives off a number of branched lateral diverticula (da.) A 

 short cloaca (klo.) leading from the stomach opens on the exterior, 

 in most cases at the posterior end of the body, sometimes on the 

 dorsal surface. There is no distinct ccelome, the space between 

 the alimentary canal and the body-wall being filled by connective 

 tissue (parenchyma), leaving only the cavities in which the sexual 

 elements are lodged. Bundles of dorso- ventral muscular fibres 

 form imperfect transverse septa, as in some Platyhelminthes. 



There is no blood-vascular system, and specialised organs of 

 respiration are likewise wanting. There is a single pair of 

 nephridia with funnel-shaped internal apertures and with external 

 openings either into the cloaca or on the surface. The nervous 



