448 /OOLOGY SECT. 



Owing to the soft character of most of their parts,' there are 

 comparal i\ely few actual remains of Chaetopoda in the older 

 geological formations, though then- arc many burrows and tracks 

 which li;i\c liccii ascribed to members of that class. Tubes of 

 tubicolnii.- I'olych.-cta have, ho\vc\cr, been found in formations 

 dating from the Cambrian period onwards. Some tubes not 

 distinguishable from those of the existing genus, Spirorbis, are 

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

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

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

 to the tubicolous Polycha-ta. The horny jaws of various Errant ia 

 have been detected in strata from the Cambrian period onwards : 

 and nian\ 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 an- known. 



APPENDIX TO THE CHJETOPODA. 



CLASS MVXOSTo.MiDA. 



The Myzostomida are a group of worms which appear to have 



(heir nearest relatives in the ('h;etopoda, though with certain 

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

 various (Yinoids, both of the stalked and the free varieties or 

 internal parasites of certain Starfishes. Thev are disc-shaped 

 animals ( Kig. 350) devoid of any trace of external segmen- 

 tation. There are patches of cilia here and there on both 

 dorsal and ventral sin-fares. At the sides there are five pairs of 

 para podia ( />), each with a chitmous hook and a supporting rod; in 

 the intervals between these there are m 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, situated at the anterior extremity, leads into a muscular 

 pharynx ( Kig. :>')[, ph) capable of being protruded as a proboscis : 

 from this a narrow (esophagus leads to the stomach, which gives off a 

 number <>f branched lateral diverticula (iff). A short cloaca (Klo) 

 leading from the stomach opens on the exterior at the posterior 

 < i nd of the body. There is no distinct ccelorne, the space between 

 the alimentary canal and the body-wall being tilled l>v connect i\ e 

 ! issue (parenchyma ), leaving only the cavities in which the sexual 

 elements are lodged. Bundles of dorso-ventral muscular fibres 

 form imperfect transverse sepia as in some I'latyhelminthes. 



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

 respiration are likewise wanting. No nephridia have been de- 

 tected. The nervous system comprises a large stellate ganglion 



