160 MANUAL OF ZOOLOGY. 



inhabits the cast-away shells of dead univalve Molluscs. The 

 different species differ much in length, varying from half an 

 inch to a foot or more. The body is cylindrical, covered by a 

 delicate cuticle, beneath which is a thick, muscular, and highly 

 contractile coat. The anterior portion of the body forms a 

 retractile trunk or proboscis, at the extremity of which is the 

 mouth surrounded by a circlet of simple tentacles. The ali- 

 mentary canal is proportionately of great length, and is much 

 convoluted. Upon reaching the posterior extremity of the 

 body it is reflected forwards, and it terminates in a distinct 

 anus which is placed anteriorly near the junction of the body 

 with the proboscis. 



In Echiurus, which is found on the coasts of the North Sea, 

 the body is provided posteriorly with zones of horny bristles ; 

 and in the Sternaspis of the Adriatic similar zones of bristles 

 are found anteriorly as well as posteriorly. 



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. Thalasfemacea. having a proboscis to which a 

 long fleshy appendage is attached. There are no oral tenta- 

 cula, and the anus is placed at the posterior extremity of the 

 body. 



CLASS II. ANNELIDA ( = Annulata). The Annelida are 

 distinguished from the preceding by the possession of distinct 

 external segmentation ; the nervous system, is composed of 

 a ventral, double, gangliated cord. 



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

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

 distinctly segmented, each segment usually corresponding 

 with a single pair of ganglia in the ventral cord. Each seg- 

 ment 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 consists of two arches, 

 termed, from their position, respectively the ' dorsal arc,' and 

 the 'ventral arc;' and each bears two lateral processes, 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 



