Lectures on the comparative anatomy and physiology of the invertebrate animals : delivered at the Royal College of Surgeons . eloped may propagate againby ova, and these two kinds of generation may alternate indefinitely :but it usually happens that the same Hydra, after having exhaustedits power of forming buds, then developes the eggs. The seas which wash our own shores are tenanted by numerousforms of minute Polypi, having essentially the same simple organisa-tion as the Hydra ; but which are protected from the dense brinyelement by an external horny integument. Now these likewisedevelope n
Lectures on the comparative anatomy and physiology of the invertebrate animals : delivered at the Royal College of Surgeons . eloped may propagate againby ova, and these two kinds of generation may alternate indefinitely :but it usually happens that the same Hydra, after having exhaustedits power of forming buds, then developes the eggs. The seas which wash our own shores are tenanted by numerousforms of minute Polypi, having essentially the same simple organisa-tion as the Hydra ; but which are protected from the dense brinyelement by an external horny integument. Now these likewisedevelope new polypes by gemmation ; but, as the external crustgrows with the growth of the soft digestive sac, the young polypeadheres to the body of the parent, and, by successive gemmations, acompound animal is produced. Yet the pattern according to whichthe new polypes and branches of polypes are developed is fixed anddeterminate in each species ; and there consequently results a par-ticular form of the whole compound animal or individual by wliichthe species can be readily recognised {Jig. 61.). This compound 126 LECTURE Campanularia dicliotoma, magnified. hydriform polype-animal, or association of polypes, resembles a minia-ture tree; but consists essentially ofa ramified tube of irritable animalmatter, f^ defended by an external,flexible, and frequently jointed, bornyskeleton, a ; and is fed by tbe activityof the tentacula, c?, and by the diges-tive powers of the alimentary sacs, g^of a hundred polypi, the common pro-duce of which circulates through thetubular cavities for the benefit of thewhole community. These currents ofthe nutrient fluid have been observedand described by Cavolini*, and morerecently by Mr. * The generaSertularia, Campanularia, Tuhu-laria, he, which form the principalsubjects of Elliss beautiful and clas-sical work on Corallines, compose thepresent division of the compound Hi/-drozoa, or hydriform polypes. The soft integument of the nutrientpolype
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