PASSIVE DEFENCE 341 



much as in the Moss-Polypes, but only some species are provided 

 with cups into which the individual members can be withdrawn, 

 nor are such cups provided with lids. The Organ-pipe Coral 

 ( Tubipora 77iusica) is another good instance. The colony is made 

 up of a number of calcareous tubes (composed of fused spicules), 

 one for each individual, and the members of the colony when 

 alarmed draw themselves back into their tubes. A common 

 British Sea- Anemone {Tealia crassicornis) gains protection by 

 means of small stones which adhere to its sticky body. When 

 the animal contracts, this stony covering not only forms a pro- 

 tection, but renders the creature very inconspicuous (see p. 289). 



Armoured Sponges and Animalcules. — The bodies of most 

 Sponges are traversed in all directions by sharp needles of lime or 

 flint, some of which may project at the surface to form a chevaux- 

 de-frise. Anyone who has incautiously handled such sponges can 

 testify to the penetrating power of these structures (see vol. i. 

 p. 485). As most persons are only familiar with the ordinary 

 bath-sponges, it may not be superfluous to repeat what has 

 elsewhere been said, that this particular kind does not possess 

 spicules, its skeleton being merely a close network of fibres of 

 horny consistency. 



Among the Animalcules {^Protozoa) protecting shells charac- 

 terize the Foraminifera and Radiolaria (see vol. i, p. 489), and in 

 the former they may be made up of foreign particles cemented 

 together, though usually calcareous in nature. The shells of 

 Radiolarians are commonly flinty, and in many cases possess 

 radiating spines. Horny shells are present in some of the 

 freshwater Rhizopods allied to the Proteus Animalcule {Amoeba), 

 and particles of sand are worked into such shells in certain 

 species (e.g. Difflugid). 



PASSIVE DEFENCE BY MEANS OF SPECIAL HABITS 



There is no sharp boundary between this and defence by 

 armour, for it often happens that such armour is only fully effec- 

 tive when disposed in a particular manner. 



A very interesting case in point is what may be termed the 

 ROLLING-UP HABIT, practised by animals of which only the upper 

 sides are properly defended. In the Three- Banded Armadillo or 

 Mataco [Tolypeutes tricinchts), for example, this can be done very 



