Mr. Lister observed similar phenomena in a species of Polyclinum, another form of com pound Ascidia, in which an excretory funnel is common to several individuals. Mr. Lister, p. 385, has adverted to the resemblance be tween the Ascidi and a zoophyte of a similar form to that here described at page 610. I may here point out an analogy on the other side, no less striking, between the Ascidi and bivalve Mollusca, in regard to the phenomena now under consideration. In both cases the water enters at one opening, and meeting with the surface of the membranous gills, passes through slits or interstices between their vessels into a space on the.other side of the gill, which space terminates at another external opening, by which the water issues. In both cases also the mar gins of the slits in the gills are fringed with cilia which exhibit a waving motion, the waves proceeding in opposite directions on the two borders of the slit. Lastly, in both cases, while the water and finer particles of matter floating in it pass through the slits, the coarser matters are conveyed along the first surface of the gills towards the mouth. The difference lies chiefly in the nature and form of the ex ternal covering and the form of the gills in each ; the membranous gills in the mussel being folded into double leaves on each side, and in the Ascidia being formed into a tubular sac ; the space between the laminm of each leaf in the mussel corresponding with the space (f) enclosed between the branchial sac and mantle in the Ascidia, both these spaces leading to the excretory orifice.
The remarkable appearances in the Mollusca described above could not wholly escape the notice of naturalists and microscopic observers. Thus we find Ant.de Heide,* a Dutch physician of the end of the seventeenth century, observing. the appearance produced by the ciliary motion in the Sea-mussel; he names it " motus radio sus," or " tremulus." He found it in most parts of the animal, but in none more evident than the gills (cirri pectinati), in which it is most easily examined. " I call the motion radiant," says he, " because it proceeds from the whole sur face of the cirrus (gill) almost in the same way as air-bubbles issue from crabstones or metals while undergoing solution ; it may be called tremulous, because the parts affected by it vibrate. This motion goes on not only in the entire gill connected with the rest of the mussel, but even in the smallest pieces cut off from it, which by their radiant motion swim briskly through the sea-water." Leeuwenhoek likewise appears, from various passages in his writings,* to have perceived the moving cilia in the Oyster and Mussel ; he noticed also the existence of the motion in 'detached portions. His observations, so far as they go, are correct; but he takes no notice of the currents in the water ; nor does he seem to have perceived the relation of the phenomenon to the respiratory or other functions, or indeed to have formed any opinion regarding its phy siological use.
Baker alludes to Leeuwenhoek's discoveries, and relates an appearance observed by himself in the Fresh-vvater Mussel, which must have been caused by the ciliary motion.f He states that " on snipping off a piece of the transpa rent membrane (gill), and viewing it with the microscope, the blood will be seen passing through numbers of veins and arteries, and if the extremity of the membrane be viewed, the true circulation or the return of the blood from the arteries through the veins will be shewn." Dr. Hales, in his Statical Essays, (vol. ii. p. 93,) plainly alludes to the same phenomena. Among more recent writers, Professor Ehrman of Berlin, in a memoir on the blood of the Mollusca, published in the Transactions of the Royal Academy of Sciences of Berlin for 1816-174 has described an appearance no ticed by him in Mya, Anodonta, the Oyster, and other Bivalves, which seems evidently to have been produced by the ciliary motion. Ile states that on viewing the inner side of the labial appendages, accessory gills, or tentacula of these Mollusca, while it was illuminated by a strong light falling in a particular direction, he perceived a very rapid' and incessant motion along the transverse stripes or furrows obser vable on the surface of the part. The motion proceeded along each stripe like a series of oscillations. It continued for some time in portions cut off from the organ. lie next ob served that a number Of round vesicular bodies escaped from the furrows or stripes at the part where they were cut, which bodies moved to and fro and as it were spontaneously in the water; and it seemed to him that in proportion as these bodies escaped, the oscillatory motion relaxed in intensity. From these facts he con cluded that the motion apparent on the surface of the part was produced by the agitation of these vesicles or animated molecules within the furrows ; that is, he supposed the furrows to be covered by a membrane to which an oscillatory motion was communicated by the agitation of the globules underneath it. He perceived the motion in question in no part but the labial appendages, and he imagined it to be connected with the male generative func tion, of which he therefore conceived the parts mentioned to be the organs. It is obvious that the appearance seen by Ehrman was the undu lating motion of the cilia, which organs, how ever, he had not recognised. He makes no mention of currents, and consequently could not perceive the connexion of the phenomenon with respiration, which was also less likely to occur to him, as he supposed the motion to be confined to the appendages mentioned.