BURROWERS AND TUBE-FORMERS 



287 



is, however, injured. 



uncertain. 1 Polydora eiliata makes a tube of mud projecting from 



the mouth of U-shaped galleries in chalk, limestone, shells, and 



even shale ; it has no hard jaws or other 



structures sufficient to account for the holes, 



but it is possible that the specially strong 



chaetae on the sixth segment may be of 



some use in this work. Other lithodomous 



worms are Sdbella saxicava and Dodecaceria 



concharum, which is a common little borer, 



forming galleries in oyster-shells, etc. 



The tubes formed by these Polychaetes 

 are very varied in constitution. 2 In some 

 cases a mucus, which hardens to form a 

 firm protective envelope, is secreted from 

 special parts (e.g. the ventral gland shields 

 of Terebellids and Sabelliformia), or from 

 the greater part of the general surface of 

 the body ; in other cases the secretion serves 

 to stick together particles of mud or sand, 

 or shelly fragments, so as to form a more or less cylindrical 

 tube (rarely branched), which is lined internally by the hardened 

 " mucus," having the appearance of silk. 



But the process of tube-making is not a simple one, for in 

 many cases, at least, the worms exhibit definite powers of choice. 

 Thus some species of Sabella choose only the very finest particles 

 of mud ; Terebella conchilega, chooses fragments of shell and grains 

 of sand ; Onupkis conchylega employs small stones more or less of 

 a size ; Sdbellaria makes use only of sand grains. Whilst some 

 worms, like Terebella, Nicomache, and others, make a very irregu- 

 lar tube, Pectinaria, builds a most remarkably neat house, 

 open at each end, which it carries about with it, the narrow end 

 uppermost (Fig. 152); the grains of sand are nearly all of the 

 same size and only one layer in thickness, embedded in abundant 

 " mucus," and with the outer surface quite smooth. 



Sir J. Dalyell 8 made some most interesting observations on 

 the method followed by sundry tube-formers in the building of 



1 Lankester has suggested that a strong acid is secreted for the purpose, see Ann. 

 Mag. Nat. Hist. (ser. 4) i. 1868, p. 233. 



2 M'Intosh, Ann. Mag. Nat. Hist. (ser. 6) xiii. 1894, p. 1. 



3 Dalyell, Tlic Powers of the Creator revealed, ii. 1853, p. 217. 



