470 ANNELIDA. 



comprises small, symmetrical, unsegmented animals, with a dis- 

 coidal body furnished below with five pairs of unjointed feet ter- 

 minated by hooks, which form cysts, somewhat like plant-galls, 

 upon the calyx, arms, or pinnules of Comatulce and other Crinoids. 

 In various fossil Crinoids the arms or pinnules have been found to 

 be distorted by the cysts of some form belonging to the Myzostom- 

 ida, and further investigation will probably show that this group is 

 really one of great antiquity. 



Annelida. 



The class of the Annelida comprises the so-called Ringed Worms, 

 including the Leeches (Hirudinea), the Earthworms and their allies 

 (Oligochceta), and the Sea-worms (Pofychcetd). The body in the 

 Annelides is always elongated, and is composed of numerous seg- 

 ments, all the segments, except those at the anterior and posterior 

 extremities of the body, being in general similar to one another. 

 In the Leeches, the segments carry no lateral appendages, and 

 locomotion is effected by means of suctorial discs. In the Oligo- 

 chsetous worms, the segments are provided with locomotive appen- 

 dages in the form of horny setae, embedded basally in the integument. 

 Lastly, in the Polychczta the segments carry, as a rule, tufts of 

 horny bristles attached to unjointed lateral protuberances or " foot- 

 tubercles " (parapodia). 



As regards their distribution in time, the Oligochceta are wholly 

 unknown as fossils, and no reliable traces of the past existence of 

 the Hirudinea have hitherto been discovered ; though some prob- 

 lematical fossils from the Lithographic Slate (Jurassic) of Germany 

 have been regarded as the remains of Leeches. The order of the 

 Polychccta, on the other hand, comprises worms which are marine 

 in habit and are at the present day very widely distributed, so that 

 we should expect to find ample evidence of their former existence. 

 As the integument of the Polychsetous worms is more or less soft, 

 and as the only hard structures developed within the tissues are 

 the horny jaws and the locomotive bristles, it can only be under 

 exceptionally favourable circumstances — as, for example, in the 

 fine-grained Jurassic lithographic slates of Germany — that the 

 actual body of these animals can have been preserved in the 

 fossil condition. Many of the Polychczta, however, protect them- 

 selves by an investing tube which may be composed of lime, or 

 of sand or other adventitious particles cemented together; and 

 the cases of these " Tubicolous " worms are often preserved. The 

 free-swimming or "Errant" Polychceta, again, have left evidence of 

 their past existence in their fossilised jaws, as well as by less un- 

 equivocal remains in the form of filled-up burrows or meandering 



