620 



POLYPI. 



transverse partitions at regular intervals for their 

 support ; as exemplified in the Tubipora Musica, 

 pi. 75, f. 34 and 66. This last figure represents 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. 75, f. 3 and 10. In 

 some species the horny base is formed into a num- 

 ber of cells, each of which answers the purpose oi 

 protecting its respective polype. These are usual- 

 ly situated at the extremity of the branches, and 

 have all the appearance of flowers, as may be seen 

 in the tabitlaria ramosa, pi. 75, f. 59, 60. The 

 different species of tlie genus Flustra, as will be 

 seen in the species carbacea, pi. 75, f. 61, and F. 

 foliacea, f. 12, have the cells excessively minute. 

 These are extended over a flat membraneous sub- 

 stance, having all the appearance of the leaves of 

 plants. These eel Is 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 polypiferous 

 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 75, f. 6, 

 is a branch of Coralium rubrum, exhibiting the ap- 

 pearance presented by the polypi in their expand- 

 ed and contracted conditions. F. 7 is the polype 

 of the same, greatly magnified. In many instances 

 the polypi are located in cup-like depressions, sit- 

 uate in the surface of the calcareous axis, which 

 protects them considerably. In the genus Madrepora 

 these depressions are intersected by radiating plates, 

 adapted to the number and construction of the ten- 

 tacula ; 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 reticulated appearance. 



The materials of which the axis is composed are 

 invariably arranged in concentric layers, thus indi- 

 cating 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, com- 

 posing a jointed structure. This has been by some 

 naturalists considered an approximation to an ar- 

 ticulated skeleton ; as it is susceptible of considerable 

 flection, and yields readily to the currents or waves, 

 without being broken. An example of this struc- 

 ture is seen in the Isis Hippuris, pi. 75, 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. 



The reproduction of all the adhesive polypi de- 

 pends upon the detachment of gemmules,* or im- 

 perfectly formed portions of their soft substance. 

 These gemmules are possessed of active powers of 

 locomotion, apparently for the sole purpose of seek- 



The word Gemmule is taken from the Latin won] gemma, 

 a bud ; and its meaning, as applied to polypi, is that of a young 

 Animal, not contained within an envelop, or egg-. 



ing a place whereon to raise its future habitation fit 

 a distance from its parents. This situation once 

 chosen, it is immovably fixed to that spot, during 

 its natural life. In the earlier state these gemmules 

 appear on the surface of the parent animal, in the 

 form of small specks, which are visible to the naked 

 eye. In the course of a few months they enlarge 

 in size, and each becomes pyriform, and are ob- 

 served to protrude from the sides of the internal 

 canals of the parent, adhering by their narrow ends. 

 This form in particular applies to the young of 

 sponges. Shortly after, they are freed, the one after 

 the other, and are borne along by the currents of 

 fluid, which are quickly passing out of the larger 

 openings. F. 73, pi. 75, represents one of these 

 detached gemmules. Were these devoid of life, 

 they would naturally sink to the bottom by their 

 own gravity, but on the contrary they spontaneous- 

 ly swim about for two or three days ; with their 

 broad end forwards. These gemmules, upon mi- 

 croscopic examination, are found to be about two- 

 thirds covered with short cilia, which are in constant 

 and rapid motion : they are extremely minute and 

 transparent, broadest at the base, and tapering to 

 almost invisible points. The strokes of these cilia 

 are made without any regular order, but conspiring 

 to propel the gemmule with the broad end forwards, 

 but without seeming to have an apparent object in 

 view, by a slow gliding motion, quite unlike the zig- 

 zag course of animals in search of prey. They ap- 

 pear, however, to have a consciousness of impres- 

 sions made on them ; for if they come in collision 

 with each other, or if they strike against any object, 

 the motion of their cilia is for a short time suspend- 

 ed ; they wheel round the spot for some seconds, 

 then renew their vibratory motion, and proceed in 

 their former course. 



It is by the narrow extremity that these gemmules 

 become adherent, which soon begins to expand it- 

 self laterally, so as to form a broad base of attach- 

 ment. While this is proceeding the cilia continue 

 to move rapidly, but their motions soon become 

 languid, and in a very few hours cease entirely to 

 move, and finally disappear, being no longer of use. 

 The same mode of proceeding is common to the 

 gemmules of all the class polypi, except that there is 

 i variety in the head of the gemmules which swims 

 irst. The time of their remaining in a free condi- 

 tion varies with the species, from a few hours to 

 three days. 



The tentacula of polypi are exquisitely sensible, 

 and are frequently seen, either singly or collectively, 

 curving their extremities towards the mouth, when 

 any minute floating body comes in contact with 

 them. During the time a polype is expanded, a 

 constant current of water is directed towards the 

 mouth; the currents are never produced by the 

 motions of the tentacula themselves, but are always 

 the effects of rapid vibrations of the cilia placed on 

 ;he tentacula. The polypi of the Flustra car- 

 basea, for example, f. 62, pi. 75, have each 

 tentacula, provided with a single row of cilia, 

 ixtending along both the lateral margins, from 

 their base to their termination ; as we have repre- 

 sented at f. 63, in a portion of one of their tenta- 

 ula, highly magnified. Every polype is furnished 

 with twenty-two tentacula, and there are about 

 fifty cilia on each side of a tentaculum, so that 

 very individual polype lias no less than two 

 thousand two hundred cilia. Every square inch 

 contains about one thousand eight hundred cells ; 

 the branches of an ordinary specimen present 

 about ten square inches of surface, so that an 

 ordinary specimen of this species presents a con- 

 regation of not less than eighteen thousand polypi ; 



