In this case the structural change, the transformation of the mesenteric filaments that occur in all Actiniæ into projectile acontia, is comparatively slight, but in another partnership between hermit-crabs and polyps the latter have undergone a much more marked adaptation. At Naples _Eupagurus prideauxii_ is one of the commonest hermit-crabs. It lives at a depth of about a hundred feet, and is often brought to the Zoological Station by the fishermen in large quantities. Its borrowed mollusc shell often bears a little polyp, _Podocoryne carnea_ (Fig. 34), which forms colonies of often several hundred individuals, arising from a common root-work of stolons which covers the shell. The polyp colony is composed of different kinds of individuals or personæ, illustrating the principle of division of labour: it includes (1) nutritive persons (_np_) which possess a proboscis, mouth, and tentacles on their club-shaped bodies; (2) much smaller blastostyles (_bl_), that is to say, polyps with degenerate mouth and tentacles, which are wholly given over to the production of buds (_mk_), which then develop into sexual animals, little free-swimming medusoids; and (3) protective personæ in the form of hard spines (_stp_), 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 personæ there are also (4) defensive polyps (_wp_) of long, thread-like shape, thickly set with stinging-cells, but possessing neither mouth nor tentacles. It might at first be 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 by the position they occupy in the colony; they are not regularly distributed 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 polyps stand in close array, sometimes spirally contracted, sometimes hanging loosely down over the hermit-crab like a fringe. Their function, like the acontia of Actiniæ, 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.