164 TIIK EVOLUTION THEORY 



several Imndrod iudividiials, arisino- from a common root-work of 

 stolons which covers tlio shell. The polyp colony is composed of 

 different kinds o\' iii(li\ iduals or persona', illustrating the principle of 

 division of labour: it includes (i) nutritive persons (li^)) which possess 

 a proboscis, mouth, and tentacles on their club-shaped bodies; (2) 

 much smaller blastostyles (hi), that is to say, polyps with degenerate 

 mouth and tentacles, which are wholly given over to the production of 

 buds (/h/.-), which then develop into sexual animals, little free-swimming 

 medusoids ; and (3) protective persona3 in the form of hard spines 

 (stj)), beneath the shelter of which the soft polyps withdraw when the 

 mollusc shell is rocked about on the sea-floor by the rolling of the 

 waves. In addition to these three different kinds of individuals or 

 persona3 there are also (4) defensive polyps (wjj) of long, thread-like 

 shape, thickly set with stinging-cells, but possessing neither mouth 

 nor tentacles. It might at first Ije thought that these are for the 

 defence of the colony, but this is not so ; the fact is that they rather 

 serve for the direct defence of the hermit-crab. This is indicated b}^ 

 the position they occupy in the colony ; they are not regularly dis- 

 tributed over the surface, but are ranged round the edge, and, indeed, 

 only on the edge which surrounds the opening of the mollusc shell. 

 Here these defensive poljq^s stand in close array, sometimes spirally 

 contracted, sometimes hanging loosely down over the hermit-crab like 

 a fringe. Their function, like the acontia of Actiniae, is to defend the 

 crab when an enemy tries to follow it within the shelter of its 

 domicile. This can easily be demonstrated by drawing out the 

 hermit-crab from the Gasteropod shell, and, when the colony has 

 settled down again, seizing the shell with the forceps and drawing it 

 slowly through the water. The water-stream which then flows upon 

 the shell mimics the attack of an enemy, and immediately all the 

 defensive polyps, as at a given signal, strike from above downwards, 

 and repeat this three or four times ; they are scaring off the supposed 

 enemy. 



In this species of polyp a special form of individual has developed 

 with a quite definite position in the colony, and furnished with a 

 special instinct or reflex mechanism which is directly useful only to 

 the crab, and has therefore, in a sense, arisen for its advantage. This 

 can quite well be explained through natural selection, for indirectly 

 these polyps are also of use to the colony, inasmuch as they protect 

 their valuable partner, and thus render it possible for the hydroid 

 colon J" to make the partnership of use to the hermit-crab as well as to 

 itself. 



This mutual arrangement thus satisfies the requirement which, 



