ANNULOSA: ANNELIDA. 179 



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). The 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 prae-oesophageal 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. 



In the higher Annelida each segment (fig. 54) consists of two 

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



Fig- 54. Diagrammatic transverse section of an Annelide. d Dorsal arc; v Ventra 

 arc ; Branchiae ; a Notopodium or dorsal oar ; b Neuropodium or ventral oar, both 

 carrying setae and a jointed cirrhus. 



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

 cesses, or "foot-tubercles" (parapodia), one on each side. 

 Each "foot-tubercle" is double, being composed of an upper 

 process, called the "notopodium," or "dorsal oar," and a lower 

 process, termed the " neuropodium," or " ventral oar." The 

 foot-tubercles, likewise, support bristles, or " setae," and a soft, 

 cylindrical appendage, which is termed the "cirrhus" (fig. 54). 



