As to the use of these motions, it inay be stated that they serve undoubtedly for renew ing the water in respiration, and probably also to convey food to the animal. Steinbuch, however, remarked that the currents were most lively in pure water, and that the extraneous matters which they conveyed seemed rattier to incommode the animal, which endeavoured to avoid them ; and from this he inferred that the currents served chiefly if not solely for respiration.
Trembley* and Bakert observed the currents produced by this polype, but both erroneously conceived them to be caused by agitation of the tentacula. Roesel I correctly remarked that, during the production of the currents, the tentacula were motionless, but riot perceiving the cilia, nor being aware that the arms when detached still produced motion in the water, he supposed that the currents were occasioned by a stream issuing from the mouth. At length Steinbuch§ discovered that separated tentacula retained the power of impelling the water; he distinguished the cilia. and their motion as the cause of the impulsion, and more correctly described the course of the cur rents : the foregoing description is in a great measure taken from his memoir, Since then several others* have made similar observations, among whom we may mention Raspail as more particularly deserving of notice, though he here, as in other cases, denies the existence of cilia.
b. Marine Polypi.—The polypi of marine Zoophytes, on which observations relating to the present subject have been made, may for our purpose be conveniently arranged under three principal forms.
The first form of polype (fig. 292) is found in Flustrw and cellular polypi generally ; it ex ists also in some spe cies which have been classed among the Ser tulari, and probably prevails very extensively in different tribes of Zoophytes. The body (a, b, e), which is gene rally contained in a cell, is bent on itself, some what like the letter Y or V; the one branch (a) being the mouth and throat, the other (b) the rectum opening by an anus, and the middle part (c),which is of a dark and often of a brown co lour, being the stomach probably with some accessory organ. The mouth is surrounded with a variable number of long straight ten tacula or arms, fringed on both of their lateral margins with cilia. When the arms are ex panded, the cilia are thrown into rapid motion, which has the appearance of undulations pro ceeding along the fringes, upwards on one side of the arm or from its root to the point, and downwards on the other. While the cilia are thus moved, they produce currents in the water, as described in the Fresh-water Polype, and here also the currents in all probability serve for respiration and the prehension of food. Besides these motions in the water in the neighbourhood of the tentacula, a revolving motion of particles is observed within the body : small particles of extraneous matter which enter the throat are moved round within it ; and the contents of the stomach and rectum undergo a very singular revolving motion round the axis of the cavity. These internal motions, Dr. Grant conjectured, might be owing to internal cilia ; and I have been able to satisfy myself of the actual existence of such internal cilia, by means of a Wollas ton's doublet of one-thirtyfifth of an inch focus ; they are very evident in the throat; in the stomach they are most distinct in the part adjoining the rectum (indicated by d in the figure), and they are clearly to be seen on the whole internal surface of the rectum (b).
I have nowhere more clearly seen the above mentioned phenomena than in a zoophyte, whose polype, though differing somewhat from the first form, may yet be referred to it. This zoophyte (fig. 293, A, B) has a creeping stem (a, a), which adheres to shells, or twines round the stems and branches of other zoophytes, (as b in the figure) ; the polypes are supported on soft pliable fleshy stalks (c), which the crea ture moves from time to time ; their body (d, and B more magnified) is bell-shaped and consists of a transparent brownish skin or envelope containing the mouth and throat (e), the stomach (g), and rectum (h). The mouth, or expanded aperture of the animal, is sur rounded by a prominent lip or border (i, i), to which the arms are attached. Cilia are distinctly visible on the arms, and within the mouth and stomach ; they are moved very briskly, and small extraneous particles indi cating currents in the water are hurried onwards towards the arms, as pointed out by the arrows at k,k; many of these particles descend along the inner side of the arms to their base, as shown by the dotted arrows o, o, o, and thence into the cavity of the mouth, from which, after being moved about for some time, the greater number are thrown out. It would seem that
the particles of food or other solid matter, after being conveyed to the inside of the arms, take then a different course from the stream of water. The latter passes inwards between the arms, and issues from the middle of the irregular circle which they form (as at m, m), carrying with it such solid matters as are not arrested on the arms; but the bodies which enter the • mouth are slowly carried along the inside of the arms (as at o, o), and in close contact with them till they reach their base. The motions of the contents of the stomach and its cilia appeared as in the Flustrm. I could perceive none in the rectum. Mr. Lister has described the same phenomena in a zoophyte closely resembling this one in the structure of the polypi, but differing in the character of the stem.* branches are formed externally of a tough (generally horny) substance, and within this of a transparent soft tissue, which is tu bular and contains a granular matter. The polypi resemble hydrve ; each is lodged in a horny cell (a, a), from which it partially protrudes itself; one orifice surrounded with tentacula serves both for receiving aliment and discharging foeces; this leads to a stomach (b), which communicates through an opening (c) at the bottom of the cell with the interior of the tubular stem and branches, the attached part or base of the polype being continuous with the soft internal tube, of which the po lypes might be regarded as a prolongation. In this form of polype, which exists in most true species of Sertularia, Campanularia, and Plumularia, and in allied genera, the tentacula or arms are destitute of cilia and incapable of giving an impulsion to the water. But a very remarkable motion has been observed by Cavolini* and Mr. Listert in the granular matter contained in the stem and branches. Although this motion has not been traced to the agency of cilia, yet as it is connected with our subject, I shall briefly notice it here. When the stem and branches of the above named zoophytes are examined with a high magnifying power, a current of granular par ticles is seen running along the axis of the tube. The current, which is compared to the running of sand in a sand-glass, after con tinuing one or two minutes in the same direc tion, changes and sets in the opposite one, in which it continues about as long, and again resumes the first, thus alternately flowing along the stem to the extremities of the branches, and back again. The change of direction is sometimes immediate, but at other times the particles are quiet for a while, or exhibit a confused whirling motion for a few seconds before the change takes place. Mr. Lister has discovered that the currents extend into the stomachs of the polypi, in which and in the mouth a remarkable agitation of particles is perceptible. When these particles are allowed to- escape from a cut branch, they exhibit, according to Mr. Lister, something very like spontaneous motion. The immediate cause of these currents is not apparent ; it seems not to be muscular contraction of the tube ; perhaps, like the agitation within the stomach, they rnay be owing to internal cilia. As to their use Mr. Lister supposes the circulating matter " to be a great agent in absorption, and to perform a prominent part in the obscure pro cesses of growth; and its flow into the stomach of the polypi seems to indicate that in this very simple family (the Sertulariw) it acts also as a solvent of the food."—Page 77. Perhaps the polypi of the Pennatula and Virgularia should be referred to this head. In these Dr. Grant* discovered a constant vibratory motion within the mouth, apparently pro duced by cilia placed round the entrance of that passage, and he saw minute particles oc casionally propelled from the mouth. Their tentacula, as in the zoophytes last referred to, did not excite currents.