TUBE-MAKERS. 327 



The size of the tube corresponds exactly to that of 

 the worm, and the animal can withdraw within it for 

 shelter. It can also turn itself in the tube, so as to 

 alter the relation of the back and belly to the sides 

 of the case. The length of the worm is about twa 

 and a half inches. 



Sabella and Serpula are names which are at least 

 known to lovers of marine life, although the animals 

 themselves are to many unknown. Sir J. G. Dalyell 

 has given an interesting account of one species, which 

 he says "is a timid, lively, active creature, whose 

 most prominent property is constructing itself an 

 artificial dwelling of the grains of comminuted sand, 

 intermingled with shelly fragments, or other indu- 

 rated substances. But there seems a great difference 

 in the solidity of the dwelling, according to the 

 position of the tube, or perhaps, the variety of 

 the architects, which has never been the subject of 

 sufficient observation. Thus we find the fabric, when 

 a cylindrical segment running over some flattened 

 surface, firm, durable, and capable of great resistance. 

 It is not easily crushed. On the other hand, when 

 cylindrical or alveolar, it appears to be always more 

 brittle. Most of the dwellings of the Sabella are 

 lined with a fine silky substance, formed of an exuda- 

 tion escaping from the body, which, consisting of 

 indurated glutinous matter, is very conspicuous on 

 breaking up the alveolar mass of some old congeries. . 



