OF CENTRAL CANADA PART IV. 249 



The Gep/iyrea, represented principally by the Sipunculus, are 

 marine, worm-like forms with thick skin. They present in some 

 respects connecting links between the worms and the holothurians, 

 whence the name of the class, from ysyvpa, a bridge. Some supposed 

 fossil forms have been cited from the Upper Jurassic Strata of 

 Bavaria. Otherwise the class is unknown in the fossil state. 



The Chcetognatha, comprise the single genus Sagitta, a small, some- 

 what fish like form from the Mediterranean, with fin- representative 

 at the posterior end of the body. Fossil forms unknown. 



The Annelida comprise earth-worms, leeches, serpulse, &c., and are 

 thus classified : 



Abranchiata without visible branchiae. 



1. Suctoria (leeches). Foss. Rep., unknown. 



2. Terricola (earth worms). Foss. Rep. unknown. 



JBranchiata with distinct branchial organs. 



1. Tubicola or C ephalo-branchiata : 



2. Errantia orDorsi-branchiata: 



The Tubicola are marine worms with a circlet of thread-like 

 branchiae around the rudimentary head. Some 

 secrete a calcareous tube or shell, and others 

 form a protecting sheath of agglutinated grains 

 of sand, &c, Two genera, Serpula, with cal- 

 careous wavy or contorted tube, and Spirorbis, 

 w i tn regularly enrolled shell (Fig, 162) are of 

 a. Serpula. b. Spirorbis. frequent occurrence in the fossil state. They 



date from the Cambrian or Silurian period, and are found mostly on 



the external or internal surface of fossil shells. 



The Errantia are marine worms with tufts of thread-like branchiae 

 along the sides of the body, and without a shell or protecting sheath. 

 They date from the Cambrian period, but many fossil examples 

 referred to the genera Ne,reites, Nemertites, &c., are of somewhat 

 doubtful nature. The narrow, cylindrical cavities found occasionally 

 in the Potsdam sandstone of the County of Leeds, Ontario, and at 

 some other localities, and which are commonly known as " Scolithus 

 cavities," are thought by some observers to have been made by boring 

 annelids, but others regard them as of fucoidal origin. Minute, horny 



