EXTINCT FORMS 



301 



chaetes are very " plastic," and can adapt themselves to changed 

 conditions of life with considerable ease ; for Nereis diversicolor, 

 Arenicola marina, and others live equally well in water of very 

 different densities, and with a different food supply. The great 

 variety in the " habitats," and presumably therefore in their food 

 supply, etc., exhibited by many Polychaetes, as well as the great 

 variation observable in some species 

 of Polynoina, and the close affinity ffl 

 of the species and genera of this | 

 sub-family, lead us to the same con- 

 clusion. 



Extinct Polychaetes. The most 

 numerous fossil records of the Poly- 

 chaetes are calcareous tubes of vari- 

 ous shapes and sizes ; they are 

 irregularly or spirally curved, and are 

 very usually attached at one end, or 

 by one surface, to stones or to fossils. 

 These tubes belong to the Serpu- 

 lidae, and are referred to the genera 

 Serpula, Spirorlis, Ditrupa, and 

 others. 1 



Spirorbis is the oldest unequiv- 

 ocal representative of the Poly- 

 chaetes, as its tubes are found more 

 or less abundantly in the Silurian 

 and other Palaeozoic strata. In 

 Palaeozoic times Serpula was rare, as 

 it was too in the Trias and Lias, but 

 in the Jurassic strata it becomes abun- 

 dant. In the chalk, S. socialis may 

 occur in masses like S. uncinata of 

 the present day, forming " Serpulite 

 chalk." In the older tertiaries the 

 genus is represented by Spirulaea. 



Terebella lapilloides occurs in the 

 Lias as a cylindrical, more or less 

 curved tube of sand-grains. 



Amongst the Nereidiformia the remains are fewer, but the 

 1 Zittel, Handbuch d. Palacontologic (Palaeozoo!ogic\ i. 1876-80, p. 562. 



. lf>9. Kunicitesavitus'Ehl. A 

 fossil worm from the lithographic 

 slate of Solenhofen : the jaws are 

 seen in front, and the ncicula 

 along each side. (From Ehlers.) 

 Natural size. 



