ANNELIDA. 89 



CLASS ANNELIDA. 



The name Worms (Yermes) is popularly applied to all the 

 diverse organisms of an elongated and segmented character, but 

 tlie lowest forms, including the so-called intestinal or parasitic 

 worms and the rotifers, are properly separated and form the 

 ]ieterogeneous class Scolecida. The higher forms, included in this 

 ■class (annelida or annulata), have distinctly segmented cylindrical 

 bodies, but without division into thorax and abdomen. When 

 lateral appendages exist they are not jointed, nor articulated to 

 the body. The nervous system is a chain of ventral ganglia; 

 and a system of tubes (pseudohsemal vessels), containing a colored 

 plasma, takes the place of heart and blood vessels. Respiration 

 ma}^ be through the skin, as in the Leech and Earthworm 

 (Abranchiata), or by means of external gills, as in the marine 

 forms (Branchiata). 



Worms have left traces in every deposit in the form of shelly 

 coverings, burrows, or imprints. Worm-holes abound in the Pots- 

 dam sandstones ; they were made at tide-level, vertically, and 

 often in pairs. The most common kind is the Scolithus linearis. 

 Many supposed annelid borings and tubes have been named, the 

 character of which are still uncertain. Serpula and Sjnrorhis 

 seem to have existed since the Upper Silurian. 



No. 171. Scolithus linearis. Hall. 



These tubes were believed by Dr. Hall to be fucoid, but they are now gen- 

 erally regarded as the borings of " Errant " annelids. The burrows are nearly 

 straight and extend vertically through the neck. From the Potsdam Sand- 

 stone, York, Penn. 



No. 173. Serpula. 



This still living genus secretes a long, shell- 

 like tube of carbonate of lime, more or less 

 tortuous, which are attached to some foreign 

 body. It is one of the " Tubiculous " anne- 

 lids; and its supposed tubes have been found 

 in all the formations above the Lower Silurian. 

 This specimen from the Quaternary, Palermo, Italy 



