'248 THE MKCHANICAL FUNCTIONS. 



is shaped into a spiral tube. Others, as the Sabella 

 and the Terebella, accomplish 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 Tere- 

 bella conchilega. These coverings, however, com- 

 posed as they are of extraneous materials, and not 

 being organic productions of the animals them- 

 selves, are structures wholly foreign to their systems. 

 These inhabitants of tubes, the Tubicoloi 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 motion 

 by the Lumhricus marimis of Linnaeus ( Arenicola 

 piscatornm of Lamarck), is very remarkable. * 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 regular cone. Each ring being so much smaller 

 than the one behind it as to admit of being received 

 within it, the whole head, when completely retracted, 

 presents a flat surface. When this disk is applied 

 to the sand, the animal, by gradually projecting the 

 cone, and successively dilating the rings of which 

 it is composed, opens for itself a passage through 

 the sand, and then secures the sides of the passage 

 from falling in by applying to them a glutinous 



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

 tions for 1826, p. 342. 



