564 



HISTORY OF ZOOPHYTES. 



holds between these and the sea-water poly- 

 pus, as between all the productions of the sea, 

 and'of the land and the ocean. The marine 

 vegetables and animals grow to a monstrous 



The reproduction of all the adhesive polypi depends 

 upon the detachment of gemmules,* or imperfectly 

 formed portions of their soft substance. These gemmules 

 are possessed of active powers of locomotion, apparently 

 for the sole purpose of seeking a place whereon to raise 

 its future habitation at 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 gem- 

 mules 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 observed to protrude 

 from the sides of the internal canals of the parent, ad- 

 hering 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. Pi. 30, f. 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 spontaneously swim 

 about for two or three days, with their broad nd for- 

 wards. These gemmules, upon microscopic examina- 

 tion, 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 con- 

 spiring to propel the gemmule with the broad end for- 

 wards, 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 appear, 

 however, to have a consciousness of impressions 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 suspended ; 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 itself 

 laterally, so as to form a broad base of attachment. 

 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 dis- 

 appear, being no longer of use. The same mode of pro- 

 ceeding is common to the gemmules of all the class 

 polypi, except that there is a variety in the head of the 

 gemmules which swims first. The time of their remain- 

 ing in a free condition 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 the tentacula. The polypi of the Flustra 

 rarbasea, for example, f. 62, pi. 30, have each tentacula, 

 provided with a single row of cilia, extending along both 

 the lateral margins, from their base to their termination: 

 as we have represented at f. 63, in a portion of one of 

 their tentacula, highly magnified. Every polype is fur- 

 nished with twenty-two tentacula, and there are about 

 fifty cilia on each side of a tentaculum, so that every 



* Tin- word Gemmule is taken from the Latin word gemma. 

 ' Had: and its meaning, ns applied t> polypi, is that of a you:i- 

 .1111111,1!, not contained within au euvelbpe'or ejjg. 



size. The eel, the pike, or the bream, of 

 fresh-waters is but small ; but in the sea they 

 grow to an enormous magnitude. The herbs 

 of the field are at most but a few feet high ; 



individual polype has no less than two thousand two 

 hundred cilia. Every square inch contains about one 

 thousand eight hundred cells ; the branches of an ordi- 

 nary specimen present about ten square inches of sur- 

 face, so that an ordinary specimen of this species pre- 

 sents a congregation of not less than eighteen thousand 

 polypi ; with three hundred and ninety-six tentacula, and 

 thirty-nine millions six hundred thousand cilia ; while 

 other species undoubtedly contain more than ten times 

 these numbers. Dr Grant has estimated that there are 

 about four hzindrcd millions of cilia on a single Flustra 

 foliacea ! the species which we have represented, f. 12. 

 We have given a representation of a gemmule of the 

 Flustra carbasea, pi. 30, f. 64. 



It is still au unsettled point whether the aggregated 

 mass is to be considered as one individual, endowed with 

 a common principle of life and growth, or whether each 

 month is to be regarded as the organ of a separate ani- 

 mal. Dr Grant is of opinion that the detached polypi 

 called pennatula, or sea pens, f. 14, pi. 30, do not pos- 

 sess a voluntary power of locomotion, but that they are 

 carried along by the currents of the ocean. Indeed, 

 none of all this extensive tribe of beings which are inves- 

 ted in a stony covering, or which have a horny or calca- 

 reous axis, have the power of locomotion ; and it is not 

 until we descend to animals divested of these, that we 

 can trace animals having this power. The fresh-water 

 polypi, called hydra, are locomotive. These animals 

 present us with the simplest kind of structure which has 

 yet been ascertained. The hydra consists simply of a 

 fleshy tube, open at both extremities, and the aperture 

 of the tube serving as a mouth, which is situate in the 

 more dilated end, and this mouth is provided at its mar- 

 gin with a single row of tentacula. Looking to this ani- 

 mal, we may suppose that nature has formed it, to prove 

 that animal life may be carried on without the aid ol 

 the complicated machinery which she has given to the 

 higher orders of creation. The hydra can change place 

 at will. F. 69, pi. 30, represents the fiydra viridis. 

 This animal has the power of fixing itself in an erect 

 position by the foot, and if it wishes to change place, it 

 slowly bends till its head touches the plane on which it 

 is moving, and adheres to it by the mouth, or by one or 

 two of its tentacula ; the foot is then detached, and by 

 a curve of the body placed close to the head, where it is 

 again fixed, preparatory to a new step, which it performs 

 by a repetition of the same movements. 



Sponges, in their general aspect, have much the ap- 

 pearance of plants, and they were by ma-ny regarded as 

 such ; but it has been satisfactorily ascertained that they 

 are composed of soft flesh, intermixed with a tissue of 

 fibres, some of which are solid, others tubular, and the 

 whole being curiously interwoven into a kind of network. 

 It will be perceived, on examining f. 70, pi. 30, that 

 every part of a living sponge presents to the eye tvo 

 kinds of orifices, the larger being somewhat round in its 

 shape, with the margins raised, forming projecting pa- 

 pillae ; the smaller being much more numerous, exceed- 

 ingly minute, and are usually termed the pores of the 

 sponge. 



For many ages, indeed, so far back as the time of 

 Aristotle, who died 322 years before the birth of Christ, 

 sponges were supposed to be so sensitive, that they 

 shrunk from the touch ; and later naturalists asserted 

 that, if punctured by sharp instruments, they would ex- 

 hibit visible tremulous motions. But Dr Grant has 

 most effectually refuted this error, by subjecting sponges 

 to the most severe experiments, such as lacerating, punc- 

 turing, burning, or otherwise wounding their texture, by 



