ANNULOSA : ANNELIDA. 205 



proboscis formed by a folded fleshy plate, susceptible of great 

 elongation, and forked at its extremity. The vent is at the 

 opposite end of the body, and the intestine is very long, and 

 folded several times. 



The British species of the class are grouped by Professor E. 

 Forbes as follows : 



Fam. I. Sipunculacea, having a retractile proboscis, at the 

 base of which the anus is placed, and round the extremity of 

 which is seen a circlet of tentacles. 



Fam. II. Priapulacea, having a retractile proboscis but no 

 tentacula, and having the anus placed at the extremity of a 

 long, filiform, caudal appendage. 



Fam. III. Thalassemacea, having a proboscis to which a long 

 fleshy appendage is attached. There are no oral tentacula, and 

 the anus is placed at the posterior extremity of the body. 



CLASS II. ANNELIDA ( = Annulata). r Y\\z Annelida are dis- 

 tinguished from the preceding by the possession of distinct 

 external segmentation ; the nervous system is composed of a 

 ventral, double, gangliated cord, with an cesophageal collar and 

 pras-cesophageal ganglion. 



This class comprises elongated, worm-like animals, in which 

 the integument is always soft, and the body is more or less 

 distinctly segmented, each segment usually corresponding with 

 a single pair of ganglia in the ventral cord. All the segments 

 are similar to one another except those at the anterior and 

 posterior extremities of the body. Each segment may also be 

 provided with a pair of lateral appendages, but these are never 

 articulated to the body, and are never so modified in the region 

 of the head as to be converted into masticatory organs. 



Fig. 77. Diagrammatic transverse section of an Annelida, d Dorsal arc ; # Ventral 

 arc ; Branchiae ; a Notopodium or dorsal oar ; b Neuropodiumor ventral oar. both 

 carrying setae and a jointed cirrhus. 



In the higher Annelida each segment (fig. 77) consists of two 

 arches, termed, from their position, respectively the " dorsal 

 arc " and the " ventral arc ; " and each bears two lateral pro- 



