ANNELIDA. 277 



only, the Serpula (Fig. 133), forms for itself an 

 external shell, which is shaped into a spiral tube. 

 Others, as the Sabella and the Terebella, accom- 

 plish the same object by collecting grains of 

 sand, or fragments of decayed shells, or other 

 substances, which they agglutinate together by 

 means of a viscid exudation, so as to form a 

 firm defensive covering, like a coat of mail. 

 Fig. 134 shows this rude architecture in the 

 Terebella conckilega. These coverings, however, 

 composed as they are of extraneous materials, 

 and not being organic productions of the animals 

 themselves, are structures wholly foreign to their 

 systems. These inhabitants of tubes, the Tu- 

 bicolce of Cuvier, are generally furnished with 

 tentacula, issuing from the head, which, when 

 the rest of the body has retired within the tube, 

 is the only part exposed. 



The expedient resorted to for progressive mo- 

 tion by the Lumbricus marinus of Linnaeus 

 (Arenicola piscatorum of Lamarck), is very re- 

 markable.* This worm, depicted in Fig. 135, 

 swarms on all sandy shores, and is dug up in 

 great numbers as bait by the fishermen. It 

 bores its way through the sand by means of 

 the peculiar construction of the rings of its head, 

 which, when elongated, has the shape of a re- 



* See the account given by Mr. Osier, Philosophical Trans-- 

 actions for 1826, p. 342. 



