The body of the Actinia, of which fig- 297 is a plan, consists entirely of a soft but tough substance, exceedingly contractile and irritable. It is usually cylindrical in shape, one end, (a, a,) named the base or foot, serving to fix the animal by adhering to rocks or other ob jects ; the other extremity is named the disc, one-half of which is seen at b, b, the other half being removed by a section ; it is sur rounded at its circumference by the arms or tentacula (c, c,) in concentric rows, and in its centre is the mouth (d), or opening of the stomach, which serves both for the entrance of food and discharge of undigested remains.
The stomach (e) is plaited longitudinally on its inside; vertical membranous partitions (g, g, g', g',) pass from its outer surface to the inside of the parietes of the body, and to the base, dividing the intermediate space into numerous compartments or cells, which com municate with each other by openings, as at ni ft b ; ao ' and also open into the tentacula, as at h. The latter are conical muscular tubes, commu nicating at their base with the cells, and open ing at, their point by a small orifice, surrounded by a sphincter muscle. The cells seem also to communicate with the cavity of the stomach, and, according to Rapp,* they open in some species by small orifices on the surface of the body. The cells and tentacula contain sea water, with which the animal can distend the whole body or any particular part of it. The protrusion of the tentacula, as is well known, is effected by their distension with water. The stomach also is often partially everted and pro truded from the mouth by an accumulation of water behind it. It has not, so far as I know, been clearly shewn by which of the communi cating orifices the water enters. Though I took considerable pains, I have not been able satisfactorily to ascertain this point ; I may remark, however, that I have repeatedly no ticed water entering at the mouth.
The ovaries and oviducts (k, k,) are lodged in the cells, and are consequently bathed in water ; of these it is unnecessary here to say more than that one part of them consists of a waving membranous fold like a mesentery, at tached by one edge to the sides of the cell, and at its free border supporting the oviduct, which resembles a. white opaque chord, termi nating, after numerous serpentine windings, in the stomach.
In regard to the 'ciliary motionin the Actinice, I am led from my observations to conclude that it exists to a greater extent in some species than in others. In all cases I have found it on the sur face of the oviducts and their supporting mem branes, which is covered with cilia of very minute size ; also on the internal surface of the sto mach, which has similar cilia, and there the currents follow the direction of the folds of the membrane. In one small but full-grown species I found currents commencing near the centre of the disc, and proceeding outwards in a radiating manner to its circumference, whence they continued along the arms as far as the points. On examining this species, which was semitransparent, by transmitted light, I distinctly perceived moving particles in the water contained within the tentacula and be hind the protruded stomach4 The motion of these particles obviously indicated a current in the water along the surfaces containing it, which current, like that on the oviducts, it may be inferred was produced by cilia, for it went on while there was no perceptible con traction taking place in any part of the ani mal. The particles indicating- the currents
within the tentacula, were moved in two diffe rent directions, namely, from the base to the point, and from the point to the base ; and (supposing the arm spread out horizontally,) the outward current was along the under part of the tube, and the returning one along the upper : (see h.) I also observed these internal currents of the tentacula in a young specimen of Actinia senilis, which seemed to have been very recently discharged from the parent; in it also there were radiating currents on the disc, but they stopped at the base of the tentacula. Thus the external currents on the disc and ten tacula were found in one species, and they occur on the disc in some other species in the young state, but their occurrence in this situa tion is by no means general in adult Actinim.
The phenomena described are in all proba bility connected with the processes of nutri tion and respiration. They bear a striking analogy to those I have observed in the Echino dermata.
The ova of the Actinim were observed by Rathke to revolve round their axis, and occa sionally to move straightforwards in the water. He could detect no cilia or other moving organs.* 6. Echinodermata. —The animals of this class in which I have observed the ciliary motion, are different species of the Sea-star (Asterias), and the Sea-urchin ( Echinus). In proceeding to describe the phenomena in the Asterias, I must first take the liberty of ex plaining some points in the anatomy of that animal, referring the reader for other details to the proper sources, especially the monograph of Tiedemann.f On the under surface of the Asterias, (I speak of the Asterias rubens in particular, ,fig. 298, A, B, C, as it is a large species and common on our shores,) we observe the mouth in the centre, and the tubular feet (1, fig. B) projecting in rows along the under part of the rays. Nearly the whole surface of the animal is beset with three kinds of eminences. First, hard calcareous processes, (a, .fig. C,) placed like studs at some distance from each other. Secondly, claw-like processes (b, b); these sin gular organs are more thickly set ; they consist of a solid stem of soft substance, bearing at the extremity a sort of pincers or forceps of hard calcareous matter, like the claw of a crab. They resemble analogous organs found on the Sea-urchin, only that the maxillw or pincers in the latter consist of three pieces ; they were named antennw or feelers by Monro, but Mul ler regarded them as parasitical animals. The third sort of processes (c, c,) are named the respiratory tubes, and are the most important in regard to our present subject. They are short, conical, membranous tubes, communi cating at their base with the internal cavity_of the body, and perforated at their point by an orifice which can be very perfectly closed. Most of them are placed in groups or patches, and, corresponding with each group of tubes, the fibrous membrane forming the wall of the body presents on its inside a pit or shallow depression (e), perforated with holes, through which the tubes communicate with the general cavity. Like the tentacula of the Actiniw, which they resemble in several other respects, they can be distended with water and elon gated, or emptied, contracted, and shortened.