600 PHILOSOPHY OF ZOOLOGY. 



in others, at each ring they are furnished with complicated 

 organs. These, in their most perfect form, may be denomi- 

 nated sheaths. They consist of a conical stalk, capable of 

 being withdrawn into the body by a kind of inclusion. This 

 sheath is furnished, at its base or extremity, withjilaments, 

 which are either simple, or jointed like the antenna? ; with 

 spines, which are solitary ; and with bristles, which are nu- 

 merous, and placed in tufts or rows. In some cases the fila- 

 ments only occur, in others the bristles or spines, and the 

 stalk is wanting ; still, however, they are retractile. These 

 lateral organs serve the purposes of feet, in burrowing in 

 mud, in crawling on its surface ; or in swimming. The 

 eyes are simple, and, in general, are numerous. 



The mouth is in the form of a sucker or proboscis, or a 

 simple aperture. In some it is unarmed, in others it is fur- 

 nished witji corneous jaws. The intestine is usually 

 straight, sometimes contracted into rings, and the anus ter- 

 minal. The organs of circulation have, in some of the 

 species, been successfully investigated, and seem to consist 

 of lateral, dorsal, and central vessels, extending the length 

 of the body, and executing the offices of veins and arteries. 

 The blood is of a purplish colour. The aerating organs 

 consist of a row of pouches pn each side, of a row of plu- 

 mose branchiae at the base of the sheathes or seated on the 

 neck, or of stellular filaments near the mouth. 



The sexes are united in the same individual, requiring, 

 however, mutual impregnation. The greater number of" 

 the species appear to be oviparous, A few are ovovivi- 

 parous. 



1st Tribe. 



Body furnished with a shell. 



A. Free, not permanently attached to other bodies. Shell 

 tiibukx conical. 



