34° 



ANIMAL DEFENCES 



(mantle), and in nearly all cases the edge of this is provided with 

 projecting bristles, which probably serve as a protection, though 

 they may also constitute a sort of filter for sifting the inflowing 

 currents of sea-water (set up by ciliary action) by which the 

 animal gets its food and oxygen. The shells of some extinct 

 lamp-shells were beset with spines. 



Echinoderms are remarkable for the calcareous plates which 

 are imbedded in the skin, and which are well seen, for instance, 

 in the ordinary Star-Fishes, Britde-Stars, and especially in Sea- 



Fig. 499.— A Sea-Urchin [Echbnis livid as), showing 

 protective covering of spines 



Fig. 500. — Part of Sea- 

 Urchin test, showing knobs 

 for attachment of spines. 



Urchins, where they are united together in a very regular manner 

 by their edges, to form a " test ". Spines may also be present, 

 and these are particularly well -developed in the Sea- Urchins 

 {Echinoids) (figs. 499, 500), which have earned their ordinary 

 name from this fact, for "urchin" is an old English word for 

 hedgehog. In some of these forms the spines (which are not 

 fixed, but united to the test by ball-and-socket joints) are so long 

 and sharp as to constitute a defence of really formidable character. 

 Armoiircd Zoophytes (Coelenterata). — Passive defence is not 

 the chief means of defence in this group, which, however, presents 

 many examples of it. The colonial branching Hydroid Zoophytes, 

 so often mistaken for sea-weeds, are invested in horny coverings 



