POLYPI. 



621 



with three hundred and ninety-six tentacnla, 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 hundred millions of cilia 

 on a single Flustra foliacea! the species which we 

 have represented, f. 12. We have given a repre- 

 sentation of a gemmule of the Flustra carbasea, 

 pi. 75, f. 64. 



It is still an unsettled point whether the aggre- 

 gated mass is to be considered as one individual, 

 endowed with a common principle of life and 

 growth, or whether each mouth is to be regarded 

 as the organ of a separate animal. Dr Grant is of 

 opinion that the detached polypi called pennatula, 

 or sea pens,f. 14, pi. 75, do not possess 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 invested in 

 a stony covering, or which have a horny or cal- 

 careous 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 loco- 

 motive. 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 

 margin with a single row of tentacula. Looking 

 to this animal, we may suppose that nature has 

 formed it, to prove that animal life may be carried on 

 without the aid of the complicated machinery which 

 she has given to the higher orders of creation. 

 The hydra can change place at will. F. 6i), pi. 

 75, represents the hydra 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 

 appearance of plants, and they were by many 

 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. 75, that 

 every part of a living sponge presents to the eye 

 two kinds of orifices, the larger being somewhat 

 round in its shape, with the margins raised, forming 

 projecting papilla; the smaller being much more 

 numerous, exceedingly 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 natural- 

 ists asserted that, if punctured by sharp instruments, 

 they would exhibit visible tremulous motions. But 

 Dr Grant has most effectually refuted this error, 

 by subjecting sponges to the most severe experi- 

 ments, such as lacerating, puncturing, burning, or 

 otherwise wounding their texture by the application 

 of corrosive chemical agents. He has discovered 

 the true nature of the currents of fluid issuing at 

 different points, which he thus graphically de- 

 scribes : " I put a small branch of the spongia 

 coalita, with some sea water, into a watch-glass, 



under the microscope, and, on reflecting the light 

 of a candle through the fluid, I soon perceived that 

 there was some intestine motion in the opaque par- 

 ticles floating through the water. On moving the 

 watch-glass, so as to bring one of the apertures on 

 the side of the sponge fully into view, I beheld, for 

 the first time, the splendid spectacle of this living 

 fountain, vomiting forth, from a circular cavity, 

 an impetuous torrent of liquid matter, and hurling 

 along, in rapid succession, opaque masses, which 

 it strewed every where around. The beauty and 

 novelty of such a scene in the animal kingdom, 

 long arrested my attention, but after twenty-five 

 minutes of constant observation, I was obliged to 

 withdraw my eye from fatigue, without having seen 

 the torrent, for one instant, change its direction, 

 or diminish, in the slightest degree, the rapidity of 

 its course." It thus appears that the large orifices 

 on the surface of a living sponge are destined for 

 the discharge of a constant stream of water from 

 the interior of the body. We have attempted to 

 represent the particles thrown out by these currents 

 in f. 70. These currents, issuing from the larger 

 orifices, are best seen by placing the living animal 

 in a shallow vessel of sea water, and strewing a 

 little powdered chalk over the surface, the motions of 

 which render the current very sensible to the naked 

 eye. It is by the myriads of minute pores, which 

 exist in every part of the surface, that this water 

 enters, conveying with it the materials necessary 

 for the subsistence of the animal. These pores 

 conduct the fluid into the interior, where, after 

 percolating through the numerous channels of com- 

 munication which pervade the substance of the 

 body, it is collected into wider passages, terminat- 

 ing in the focal orifices above described, and is 

 finally discharged. The mechanism by which these 

 currents are produced is involved in much ob- 

 scurity. 



The genus vorticella is constituted of a small 

 tribe of animals which differ from the polypi in one 

 particular, namely, that of being destitute of tenta- 

 cula, and having cilia only, surrounding the margin 

 of a bell-shaped body, which is mounted upon a 

 long, slender peduncle. The animals of this 

 genus are always attached to some extraneous body 

 by this foo\sta\k,asrefiresented\n vorticella polypina, 

 pi. 75, f. 75, but have the power of moving about 

 in all directions, to the extent of the range of the foot- 

 stalk. Currents are, as usual, excited by the vibra- 

 tions of the cilia, and these are the efficient instru- 

 ments of progressive motion. The ordinary position 

 of the peduncle of the vorticella is spiral, but it 

 can extend it quite in a straight line when in search 

 of food ; but it suddenly retreats from danger, by 

 resuming the spiral folds of its peduncle. 



Lamarck arranges the polypi under five orders, with the 

 following characters : 



I. POLYPI NATANTES. Polypi provided with tentacula, 

 united in a common fleshy mass, placed on an axis, free, and 

 floating in the water. 



II. POLYPI TUBIFERI. Tentaculated polypi united in a 

 common fleshy body, without any solid axis, and covered with 

 tubiform cylinders. 



III. POLYPI VAGINATI. Polypi with tentacnla, always 

 fixed in an inorganic, covering, and forming in general com- 

 pound animals. 



IV. POLYPI DENUDATI. Tentaculated polypi not forming' 

 a common envelop, fixed either constantly or spontaneously. 



V. POLYPI CILUTI. Polypi without any tentacula; but 

 instead of them, vibratile cilise, at or near the mouth. 



ORDER I. POLYPI NATANTES. 



Polypi united in a common fleshy body, or congregated 

 mass, free, elongated, enveloping an inorganic axis, cartiliiui- 

 nous, osseous, or stony ; each polypus provided with tenta- 

 cula placed around the mouth, and radiating. 



1 he animals of this order are congregated on a common 

 tidily, in which they all participate, while each appears to 

 enjoy a separate existence, aud distinct powers of volition. 



