PASSIVE DEFENCE 339 



sufficiently well-developed and numerous to constitute a means 

 of defence, as, for instance, in the Sea-Mouse (Aphrodite aculeata) 

 (p. 147) and the Porcupine Worm (Hermione hystrix). The 

 tube-dwelling worms, which are closely allied to the preceding, 

 construct habitations of the most varied kind, which may either 

 (like the shells of Molluscs) be entirely secreted by their pos- 

 sessors, or may be made up of foreign bodies firmly cemented 

 together. We may contrast the long parchment-like tube of 

 HyaKncecia with the curved or undulating calcareous tubes of 

 Serpula (see p. 258) and its allies, and the spiral house of Spiror- 

 bis made up of similar material. In these forms, too, one of the 

 tentacles carried on the head is converted into a stopper or oper- 

 culum, by which the tube is closed when the animal is withdrawn. 

 The most varied foreign bodies are used for tube-construction, 

 nor are these necessarily associated in a haphazard manner. Of 

 this matter Benham (in The Cambridge Natural History] says: 

 " 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; Onuphis conchilega employes small stones more or less 

 of a size; Sabellaria makes use only of sand grains. Whilst 

 some worms, like Terebella, Nichomache, and others, make a very 

 irregular tube, Pectinaria builds a most remarkably neat house, 

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

 uppermost; 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." 



Moss- Polypes (Poly zoo) are for the most part colonial animals, 

 collectively forming a mass of the most varied shape, flattened 

 (" sea-mats "), shrub-like, as encrustations on sea-weeds, &c. The 

 members of the colony are invested by horny coverings, some- 

 times hardened by calcareous deposits, and each of them can be 

 withdrawn into a little cup, which is in some cases closed by a lid 

 or operculum. The defences of the colony may be strengthened 

 by the addition of projecting spines. 



Lamp- Shells (Brachiopoda) are enclosed in bivalve shells, but 

 these are quite different in character from those possessed by 

 bivalve molluscs, and are also differently placed in relation to the 

 body (see vol. i, p. 439). Each valve is lined by a fold of skin 



