POLYPI. 



619 



thus be joined together artificially, so as to compose 

 living monsters more complicated than the wildest 

 fancy has conceived. Most, of the polypi form com- 

 pound animals, attached to one another by lateral 

 appendages, or by their posterior extremity, partici- 

 pating in a common life, while at the same time they 

 enjoy their individual and independent existence. 

 Many of this tribe are supposed to he gemmiferous, 

 or to extend the race by buds in the same manner 

 as plants, while others propagate their species by 

 means of eggs. In the lowest races of polypi, the 

 distinctive characters of animal life are so slightly 

 developed, that there is much difficulty in distin- 

 guishing them from the cryptogamic families of 

 the vegetable kingdom. The resemblance of these 

 animals to plants consists in this, that from the egg 

 is formed a bulb, which shoots up into a stem, and 

 sends off branches ; there is also a root, which, 

 however, is merely the organ of attachment, afford- 

 ing no nourishment to the animals. Being thus 

 immovably fixed to a particular spot, they have 

 no other means of providing themselves with food, 

 but by their long tentacula, which act as arms to 

 convey the food to the mouth ; these members 

 only are capable of voluntary motion. The vege- 

 table structure of this class, long obtained for them 

 the name of animal plants. Some of them, how- 

 ever, float about in the ocean as the pennatula, or 

 sea pens. 



About the year 1699, Imperati and Gesner had 

 remarked the animal structure of polypi or corals; 

 and Peysonnell, in 1727, was the first who ascer- 

 tained the living inhabitants of those stony and 

 horny structures, and his discoveries have been 

 confirmed by Trembley's treatise upon the hydra;, 

 published in 1740 ; and subsequently by Reaumur, 

 Jussieu, Donati, Ellis, Boccone, Degeer, Baster, Ca- 

 volini, Pallas, Linnaeus, and Cuvier. All the animals 

 of this class were placed by Linnaeus, as an order of 

 his class formes, under the distinctive appellation 

 of Lithophytae. The sagacity of that great natura- 

 list, enabled him to form a superstructure, upon 

 which has been built the more improved, because 

 better known classifications of Pallas, Bruguiere, 

 and Lamarck ; whose arrangement we mean to 

 follow, as being more comprehensive than that of 

 Cuvier, who, divides his class polypi into two 

 orders ; the first of which embraces those that are 

 naked, and the second, such as live in polypiferous 

 masses, formed by the united labours of the com- 

 munity. These he subdivides into many families. 



Minute as the beings are which construct and 

 inhabit those stony masses called corals, they form 

 one of the largest, and undoubtedly the most singu- 

 lar of the whole classes of animated being. Such 

 is the enormous accumulation of the stony enve- 

 lops formed by them in tropical seas, that islands 

 are produced, coasts extended, and harbours blocked 

 up by them. It was the opinion of Lamarck, that 

 it was these minute beings who originally formed 

 the calcareous strata of the globe. 



In contemplating the structure of the polypi, the 

 hydra for example, we find their nutritive organs 

 the simplest of all possible forms ; consisting of a 

 mere stomach adapted to receive and digest food, 

 without any other apparent organ, being destitute 

 of brain, nerves, or organs 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. 75, f. 79, exhibit- 

 ing 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 trans- 



udation through its substance, probably by means of 

 invisible pores, of the nutritious particles thus ex- 

 tracted from the food for the purpose of its being 

 incorporated and identified 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 inside 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 hap- 

 pened. 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 wonder- 

 ful discovery was subsequently confirmed by Bon- 

 net, 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 present 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 cur- 

 rents of water to flow towards their mouths, carry- 

 ing 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 directions from the cavity of each sto- 

 mach, dividing into many branches, and being dis- 

 tributed over all the surrounding portions of the 

 flesh. These branches communicate with similar 

 channels, proceeding from the neighbouring sto- 

 machs, so that the food which has been taken in by 

 one of the mouths, contributes to the general nour- 

 ishment 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 succession on the nutritive juices, and, after 

 performingthis office, may be expelled, and exchang- 

 ed for a fresh supply. It has accordingly been 

 conjectured, on the presumption that this function 

 is equally necessary to them as it is to all other 

 animals, that the vivifying influence of the surround- 

 ing element, is exerted through the medium 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 accom- 

 plishing this function has been distinctly traced in 

 the animals of the order Acalepha, which are treat- 

 ed of under the article Echinodermata. 



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 



