ANNELIDA. 449 



foreign materials, such as sand, small stones, and the 

 debris of shells, lined internally with a smooth coating of 

 hardened mucus ; in others it is of a leathery or horny 

 consistency ; and in some it is composed, like the shells of 

 Mollusca, of calcareous matter secreted by the animal. 

 The Tubicola generally live in societies, winding their 

 tubes into a mass which often attains a considerable size : 

 a few are solitary in their habits. They retain their posi- 

 tion in their habitations by means of appendages very 

 similar to those of free worms, with tufts of bristles and 

 spines ; the latter, in the tubicular Annelides } are usually 

 hooked ; so that, by applying them to the walls of its 

 domicile, the animal is enabled to oppose a considerable 

 resistance to any effort to draw it out of its case. In the 

 best known family of the order (Sabellidce), the branchiee 

 are placed on the head, where they form a circle of 

 plumes or a tuft of branched organs. The Serpulce form 

 irregularly twisted calcareous tubes, and often grow 

 together in large masses, when they secure themselves to 

 shells and similar objects j other species, Terebella, which 

 build their cases of sand and 

 stones, appear to prefer a 

 life of solitude. The curious 

 little spiral shells seen upon 

 the fronds of sea-weeds, are 

 formed by an animal be- 

 longing to the family Spi- 

 rorbis. 



If the animals are placed 

 in a vessel of sea-water, a 

 very pleasing spectacle will 

 soon be witnessed. The mouth 

 of the tube is first seen to 

 open, by the raising of an 

 exquisitely-constructed door, 

 and then the creature cauti- 

 ously protrudes the anterior Fig . w._ ASerpulapro1rudedfromits 

 part ot its body, spreading calcareous tube. 



out at the same time two beautiful fan-like expansions, of a 

 rich purple, or scarlet colour, which float elegantly in the sur- 

 rounding water, and serve as branchial or breathing organs. 



GG 



