i;8 MANUAL OF ZOOLOGY. 



viously annulated, sometimes not ; but there are no ambulacral 

 tubes nor foot-tubercles, though there are sometimes bristles 

 concerned in locomotion. The nervous system consists of an 

 oesophageal nerve-collar and a cord placed along the ventral 

 surfags of the body. 



The Sipunculus, and its allies (fig. 53), make up this class, 

 and from their affinity to the worm-like Holothurians they have 

 often been placed amongst the Echinodermata. They are not, 

 however, provided with an ambulacral system, the integument 

 is not capable of secreting calcareous matter, and there are no 

 traces of any radiate arrangement of the nervous system. 



Fig. 53. Gephyrea. Syrinx nudus. (After Forbes.) 



The Sipunculus is a worm which is found burrowing in the 

 sand of the coasts of most of our European seas, or which in- 

 habits 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. The sexes are distinct in all the Gephyrea, 

 and the young pass through a metamorphosis. 



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. In the Echiurida, 

 also, there are branched tubes connected with the termination 

 of the intestine, which are doubtless homologous with the 

 "respiratory tree" of the Holothurians. 



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

 Forbes as follows : 



