142 AN INTRODUCTION TO THE STUDY OF FOSSILS 



nephridia, a pair of nerve ganglia from the ventral chain and 

 blood vessels. The nervous system consists of a dorsal brain 

 connected around the mouth with the anterior end of the 

 double ventral nerve cord. Paired excretory organs (nephridia) 

 conduct the nitrogenous waste matter from the general body 

 ca\aty (coelome) to the exterior. 



Derivation of name. — Latin annulus, a ring, referring to 

 the ring-like constrictions of the body. 



The annulata are divided into the four following classes : 



A. Archi-annelida Living. 

 A primitive class of annulata. Unknown in the fossil state. 



B. Hirudines Living. 

 Including the genus Hirudo, the leech. Unknown fossil. 



C. Gephyrea Living. 

 With larva similar to that of the Chaetopoda. Unknown 



fossil. 



D. Chaetopoda Cambrian to present. 



CLASS D, CH^TOPODA 



Type of the class, Nereis virens (Fig. 55). 



This form lives in sandy or muddy beaches, usually between 

 tide levels, under rocks or among seaweeds ; at times it reaches 

 a length of over eighteen inches. It inhabits a burrow which 

 it makes firm by cementing the sand particles of the wall to- 

 gether with a mucilaginous secretion. This common form 

 ranges from Long Island to Labrador, to Great Britain and 

 Norway. 



Its body, rounded above and flattened below, is divided by 

 ring-like constrictions into a series of segments. Each segment, 

 except the most anterior (head segment) and most posterior 

 (tail segment), bears externally at the sides of the body, a pair 

 of outgrowths, — the flapper-like gill-feet or parapodia, which 

 aid in creeping or act as oars in swimming. The parapodia are 

 strengthened by chitinous rods extending outwards from the 

 body and are bordered by several bundles of chitinous bristles. 



