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which consists of a simple digestive, or stomach tube, open at both ends, and suspended in the centre of the polyp by little partitions called mesenteries; indeed, the resemblance to an ordinary zoantharian is so close that we cannot feel any surprise when Dr. Percival Wright tells us that in living specimens brought up from the deep-sea soundings at Setubal, he has observed the little wart-like projections of the bark expanding their tentacles in search of food just like the common sea anemone of our aquaria, or any other zoantharian. It is a compound polyp; that is, although each little nipple-like projection has its own digestive sac, and its own prehensile organs or tentacles by which it secures its own food, they all unite together to contribute to the growth of the compound mass of which they form part. Such being the case, it is clear that the polythoa, or coriaceous investment of the glass rope, must be an organism totally distinct from the sponge mass, whose nutrition we have already seen to be derived from the currents of water flowing through its aquiferous channels. If we admit the silicious stem or glass rope to be an essential part of the spongy portion, we shall be constrained to regard the polythoa as a mere parasitic investment, and having no structural relations with the sponge and its glass coil at all. There are still, however, some observers who consider that from the universality with which the bark is found coating the glass rope and no where else, and from its containing peculiar silicious spicules, that these two, the polythoa and glass rope, form one organism which may exist independent of the sponge at all, as was first supposed, and they consider such specimens as these, in which no trace can be discovered of the spongy expansion, and the glass coil is coated to its extremity with polythoa, as strongly favouring this belief. It is perhaps premature to pronounce dogmatically either way, although the first of these views—that is, of the parasitic nature of the polythoa—meets with almost universal acceptance.
What, then, are these spicules of which so frequent mention has been made? They are easily obtained for
