THE POLYPUS. 



563 



which they are all possessed, and with which 

 they have a slow progressive motion ; but the 

 moderns have given the name of Polypus to a 

 reptile that lives in fresh water, by no means 



apparent organ, being destitute of brain, nerves, or or- 

 gans of sense ; nor is there the slightest appearance of 

 any thing corresponding to lungs, heart, or even vessels 

 of any kind. We have given a magnified representation 

 of the hydra, laid open by a longitudinal section, pi. 30, 

 f. 79, exhibiting the cavity into which the food is received 

 and digested. The walls of this cavity must be adapted 

 not only to prepare and pour out the fluids by which the 

 food is digested, but also to permit of the transudation 

 through its substance, probably by means of invisible 

 pores, of the nutritious particles thus extracted from the 

 food for the purpose of its being incorporated and identi- 

 fied with the gelatinous pulp, of which the body appears 

 wholly to consist. The researches of Trembley have 

 brought to light the extraordinary fact, that not only the 

 internal surface of the polypus is endowed with the 

 power of digesting food, but that the same property be- 

 longs also to the external surface, or what we might call 

 the skin of the animal. He found that by a dexterous 

 manipulation, the hydra may be completely turned in- 

 side out, like the finger of a glove, and that the animal, 

 after having undergone this singular operation, will very 

 soon resume all its ordinary functions, just as if nothing 

 had happened. It accommodates itself in the course of a 

 day or two to the transformation, and resumes all its natural 

 habits, eagerly seizing animalcules with its tentacula, and 

 introducing them into its newly formed stomach, which 

 has for its interior surface what before was the exterior 

 skin, and which digests them with perfect ease. The 

 truth of this wonderful discovery was subsequently con- 

 firmed by Bonnet and Spallanzani. 



Still more complicated are the forms and economy of 

 the aggregated polypi, which prolific nature has spread 

 in countless multitudes, over the rocky shores of the 

 whole globe. These grow in the form of plants, and are 

 supported on one common stem, with widely extended 

 flowering branches. These many-headed monsters pre- 

 sent myriads of open mouths, each surrounded by single 

 or numerous rows of tentacula, which are extended to 

 catch their prey; these are provided with a multitude of 

 cilia, which, by their incessant vibrations, determine 

 currents of water to flow towards their mouths, carrying 

 with them the floating animalcules on which the entire 

 mass of polypi subsists. 



Each mouth leads into a separate stomach, whence the 

 food, after its digestion, passes into several channels, 

 generally five in number, which proceed in different di- 

 rections from the cavity of each stomach, dividing into 

 many branches, and being distributed over all the sur- 

 rounding portions of the flesh. These branches commu- 

 nicate with similar channels, proceeding from the neigh- 

 bouring stomachs, so that the food which has been taken 

 in by one of the mouths, contributes to the general nou- 

 rishment of the whole mass of aggregated polypi. 



The polypi appear in general not to be provided with 

 any distinct channels for conveying aerated water into 

 the interior of their bodies, so that it may act in succes- 

 sion on the nutritive juices, and, after performing this 

 office, may be expelled, and exchanged for a fresh sup- 

 ply. It has accordingly been conjectured, on the pre- 

 sumption that this function is equally necessary to them 

 as it is to all other animals, that the vivifying influence 

 of the surrounding element, is exerted through the me- 

 dium of the surface of the body. Thus it is very possible 

 that in polypi, while the interior surface of the sac di- 

 gests the food, its external surface may perform the office 

 of respiration ; and no other mode of accomplishing this 

 function has been distinctly traced in the animals of the 

 order acalepha. 



so large or observable. These are found at the 

 bottom of wet ditches, or attached to the under 

 surface of the broad-leafed plants that grow 

 and swim on the waters. The same difference 



The form of the shelly covering which invests most of 

 the polypi, admits of almost infinite variety. In some it 

 encloses the flesh in a general sheath, leaving only an 

 opening at the end, sufficient for the expansion of each 

 set of tentacula, surrounding the various mouths of the 

 respective animals. In some species these tubes are 

 placed parallel to each other, in the manner of the pipes 

 of an organ, with transverse partitions at regular inter- 

 vals for their support ; as exemplified in the Tubipora 

 Musica, pi. 30, f. 34 and 66. This last figure repre- 

 sents a portion of the tubes highly magnified, and laid 

 open to show the polypi in their interior. Sometimes 

 the tubes are united together endwise, like the branches 

 of a tree, leaving lateral apertures for the animals to 

 extend their tentacula through as exemplified in the 

 sertularia frutescens, pi. 30, f. 4 and 10. In some 

 species the horny base is formed into a number of cells, 

 each of which answers the purpose of protecting its re- 

 spective polype. These are usually situated at the ex- 

 tremity of the branches, and have all the appearance of 

 flowers, as may be seen in the tabularia ramosa, pi. 30, 

 f. 59, 60- The different species of the genus Flustra, 

 as will be seen in the species carbacca, pi. 30, f. 61, and 

 F. foliajcea. f. 12, have the cells excessively minute. 

 These are extended over a flat membraneous substance, 

 having all the appearance of the leaves of plants. These 

 Cells are formed in very regular rows, as in the magnified 

 representation, of F. foliacea, f. 1, and are arranged 

 with as much regularity as the cells of a honeycomb. 

 Nearly the whole of the animals which constitute the 

 other polipiferous masses have an internal inorganic base 

 of support, constituting a kind of skeleton or axis ; the 

 mouths of the polypi being developed at intervals over 

 the surface of the fleshy layer by which this skeleton is 

 covered. This is especially the case with the genera 

 Gorgonia, Antipathes, and Corallium, as they bear the 

 strongest similitude to the branched forms of the stems 

 of vegetables. The flesh contains grains of calcareous 

 matter, which, in the dried specimens, adhere to the 

 surface of the stems. Plate 30, f. 6, is a branch of 

 Coralium rubrum, exhibiting the appearance presented 

 by the polypi in their expanded and contracted condi- 

 tions. F. 7 is the polype of the same, greatly magnified. 

 In many instances the polypi are located in cup-like de- 

 pressions, situate in the surface of the calcareous axis. 

 which protects them considerably. In the genus Madre- 

 pora, these depressions are intersected by radiating 

 plates, adapted to the number and construction of the 

 tentacula ; and in the Millepora the cells are closer and 

 more minute, and do not exhibit any of the star-like 

 radiations. In some species the plates have more of a 

 parallel arrangement ; and in others they form a reticu- 

 lated appearance. 



The materials of which the axis is composed are inva- 

 riably arranged in concentric layers, thus indicating that 

 their deposition has been successive, and the surface is 

 always marked by longitudinal lines corresponding to the 

 figure of the animal covering the flesh. In some genera 

 the stem consists of horny and calcareous parts alternately 

 disposed, composing a jointed structure. This has been 

 by some naturalists considered an approximation to an 

 articulated skeleton ; as it is susceptible of considerable 

 flection, and yields readily to the currents or waves, 

 without being broken. An example of this structure is 

 seen in the Jsis Hippuris, pi. 30, f. 21. 



Almost the entire class of polypi are attached, by the 

 root of the stem or base, to submarine rocks or other 

 extraneous bodies. The roots are possessed of a very 

 strong adhesive quality. 



