The third form of polype is found in Tubularia. Fig. 295 represents a magnified view of a common species, the Tubularia indivisa. There is a transparent horny tube (a, a), containing a soft mat ter, which at the extremity of the tube is continuous with the stomach (b) and the mouth (c). There are two rows of tentacula or arms, one (d) immediately surrounding the orifice of the mouth, the other (e) further back, be tween the mouth and stomach.
The arms are destitute of cilia and excite no movement in the water; but Mr. Listert has discovered a remarkable motion of particles within the tube, which has some re semblance to the circulation of globules observed in plants of the genus Chara. These particles moved in a current within the tube, the general course of the stream being parallel to the slightly spiral lines of spots on the tube, and in the directions marked by the arrows. On the greater part of the side first viewed (the one represented) it set as from the poly pus ; but on the other side the flow was to wards the polypus, each current thus occupy ing half the circumference. The tube had a granulated appearance between the lines of spots, and beneath this the particles ran. Their course was even and uniform without any starting or dancing motion, such as is observed in the Sertulariw. At the nodous parts of the tube (m, n) were slight vortices in the current, and at o near the end of the tube it came over from the opposite side. Two currents were continually going on in the mouth and the stomach, one always flowing down the sides in the direction e, e, and the opposite one in the axis. Neither the cause of these currents nor their use has been ascertained.
Such are the phenomena of the ciliary and other apparently allied motions in the Marine Polypi.
Spallanzani seems to have first noticed them ; he observed the currents produced by the Flustra., but erroneously attributed them to the agitation of the arms, the cilia on which he had not perceived. Dr. Fleming* described the current along the tentacula in the Valkeria cuscuta (a genus which he has separated from the Sertularize, among which it was previously included,) and distinguished the cilia with their undulatory motion. Dr. Grantt discovered the cilia on the arms of the Flustrze and described their undulatory motion, to which he ascribed the motion in the water. He also pointed out the
revolving motion of particles within the mouth.; stomach, and rectum, and conjectured that it was owing to the action of internal cilia, which conjecture I have been able tef verify. Dr. Grant also discovered the vibratory and probably ciliary motion within the mouth of the polype of the Pennatulm. Loeflingl first observed the agitation of granular matter within the stem and branches of the Sertu larize. Cavolini afterwards more correctly de scribed this as a current of fluid holding granules in suspension-, running first in one direction and then in the other. Lastly, Mr. Lister observed anew these internal currents of the Sertulariw, described them more mi nutely, and showed that they extended into the stomach of the polypes. Mr. Lister has also described the phenomena in the Flustrw previously observed by Dr. Grant. Ile dis covered the currents within the stem of the Tubularia, which, as far as I know, had not been previously noticed.
c. the various species of sponges, water, the element in which they live and grow, passes in currents through pores and canals in their substance, in a con tinuous manner, entering at one place and issuing at another. This phenomenon has not been directly traced to the agency of cilia; it comes nevertheless to be considered here, as such an agency is highly probable, and at least the motion of the water is not owing to any contraction of the canals in which it flows, but is obviously caused by some other kind of impulsion communicated to it by the surface along which it passes.
In a common sponge we see a number of pretty large orifices on the surface, each opening on the summit of a conical eminence or pa pilla (fig. 296, a). These openings are named by Dr. Grant the " fcal orifices." Innume rable small pores occupy the rest of the surface, and give to it its peculiar character. These pores penetrate to a certain depth, and lead into canals (b), which, uniting together and gradually growing larger, terminate in wide tubes, which open at the fzecal orifices. The pores, excretory canals, and fwcal orifices thus form continuous passages through the sponge. In the fresh state they are lined throughout with a smooth gelatinous coating.