ACTIVE DEFENCE 



361 



Is an absence of observations on the subject. At any rate, many 

 of them are extremely sharp-pointed, and often serrated in such 

 a way as to render their possessor a somewhat unpleasant morsel 

 of food. 



Many of the Planarian Worms, when irritated, discharge large 

 numbers of microscopic rods (rhabdites] from the skin, and it is 

 likely that these possess irritant properties. Some few members 

 of the same group are also provided with stinging- or nettling-cells, 

 much like those of sea-anemones, jelly-fish, 

 and similar creatures, and these are un- 

 doubtedly defensive structures. 



Among Echinoderms we find that some of 

 the Sea -Urchins are provided with poison- 

 spines (fig. 506), reminiscent of what has been 

 described for certain fishes (see p. 355). Such 

 a spine has a swollen end in which a poison- 

 bag is lodged, this communicating with an 

 excessively-sharp perforated style, by which 

 the wound is inflicted and poison introduced 

 into it. 



The innumerable nettling- or stinging-cells 

 with which the members of the great phylum 

 Ccelenterata (jelly-fishes, sea-anemones, corals, 

 <&c.) are provided undoubtedly serve as actively- 

 defensive weapons, though perhaps it is right 

 to primarily regard them as means whereby 

 active prey is paralysed and secured (see 

 p. 158). The brightly-coloured Sea- Anemones 

 are richly endowed with these protective struc- 

 tures, and it is a matter of observation that most fishes leave 

 them severely alone. Probably the vivid hues which they flaunt 

 are to be looked upon as examples of " warning-coloration", and 

 the same thing is very likely true for Corals. But, as already 

 remarked, every means of defence is more or less met by counter 

 devices among aggressive forms, and to some of the coral-reef 

 fishes the stinging -cells have no terrors. The Parrot -Fish 

 (Scarus), for instance, browses upon corals, the hard parts of which 

 are effectively tackled by its firm parrot-like jaws, while the sting- 

 ing-cells are not able to injure the hard lining of its mouth. 



The Slipper-Animalcule (Paramcecium] is a good example of 



Fig. 506. Poison-spine of a 

 Sea-Urchin (Asthenosoma urens) 

 in longitudinal section: p,p, holes 

 in side of the hollow spine; tn, 

 layers of muscle. Diagrammatic 

 and enlarg 



