624 CILIA. 



the right and descending on the left of each oval, face of the cirrus (gill) almost in the same way 



as viewed from without ; but the cilia them- as air-bubbles issue from crabstones or metals 



selves are very much closer than the apparent while undergoing solution ; it may be called 



teeth, and the illusion seems to be caused by a tremulous, because the parts affected by it 



fanning motion given to them in regular and vibrate. This motion goes on not only in 



quick succession, which will produce the ap- the entire gill connected with the rest of the 



pearance of waves, and each wave answers mussel, but even in the smallest pieces cut off 



here to a tooth. from it, which by their radiant motion swim 



Whatever little substances alive or inanimate briskly through the sea-water." 

 the current of water brings, if not ejected as Leeuwenhoek likewise appears, from various 



unsuitable, lodge somewhere on the surface of passages in his writings,* to have perceived the 



the branchial sac, along which each particle moving cilia in the Oyster and Mussel ; he 



travels horizontally with a steady slow course noticed also the existence of the motion in 



to the front of the cavity, where it reaches a detached portions. His observations, so far as 



downward stream of similar materials (h') ; they go, are correct ; but he takes no notice of 



and they proceed together, receiving accessions the currents in the water ; nor does he seem to 



from both sides, and enter at last, at the have perceived the relation of the phenomenon 



bottom, the oesophagus (h) ; this is a small to the respiratory or other functions, or indeed 



flattened tube which carries them, without any to have formed any opinion regarding its phy- 



effort of swallowing, towards the stomach. siological use. 



Mr. Lister observed similar phenomena in a Baker alludes to Leeuwenhoek's discoveries, 

 species of Polyclinum, another form of com- and relates an appearance observed by himself 

 pound Ascidia, in which an excretory funnel is in the Fresh-water Mussel, which must have 

 common to several individuals. Mr. Lister, been caused by the ciliary motion.f He states 

 p. 385, has adverted to the resemblance be- that " on snipping off a piece of the transpa- 

 tween the Ascidiae and a zoophyte of a similar rent membrane (gill), and viewing it with the 

 form to that here described at page 610. I may microscope, the blood will be seen passing 

 here point out an analogy on the other side, no through numbers of veins and arteries, and if 

 less striking, between the Ascidiae and bivalve the extremity of the membrane be viewed, the 

 Mollusca, in regard to the phenomena now true circulation or the return of the blood from 

 under consideration. In both cases the water the arteries through the veins will be shewn." 

 enters at one opening, and meeting with the Dr. Hales, in his Statical Essays, (vol. ii. 

 surface of the membranous gills, passes through p. 93,) plainly alludes to the same phenomena, 

 slits or interstices between their vessels into a Among more recent writers, Professor Ehrman 

 space on the other side of the gill, which space of Berlin, in a memoir on the blood of the 

 terminates at another external opening, by which Mollusca, published in the Transactions of the 

 the water issues. In both cases also the mar- Royal Academy of Sciences of Berlin for 

 gins of the slits in the gills are fringed with 1816-17, \ has described an appearance no- 

 cilia which exhibit a waving motion, the waves ticed by him in Mya, Anodonta, the Oyster, 

 proceeding in opposite directions on the two and other Bivalves, which seems evidently to 

 borders of the slit. Lastly, in both cases, have been produced by the ciliary motion. He 

 while the water and finer particles of matter states that on viewing the inner side of the 

 floating in it pass through the slits, the coarser labial appendages, accessory gills, or tentacula 

 matters are conveyed along the first surface of of these Mollusca, while it was illuminated by a 

 the gills towards the mouth. The difference strong light falling in a particular direction, he 

 lies chiefly in the nature and form of the ex- perceived a very rapid and incessant motion 

 ternal covering and the form of the gills in along the transverse stripes or furrows obser- 

 each; the membranous gills in the mussel vable on the surface of the part. The motion 

 being folded into double leaves on each side, proceeded along each stripe like a series of 

 and in the Ascidia being formed into a tubular oscillations. It continued for some time in 

 sac; the space between the laminae of each portions cut off from the organ, lie next ob- 

 leaf in the mussel corresponding with the served that a number of round vesicular bodies 

 space (f) enclosed between the branchial sac escaped from the furrows or stripes at the part 

 and mantle in the Ascidia, both these spaces where they were cut, which bodies moved to 

 leading to the excretory orifice. and fro and as it were spontaneously in the 



The remarkable appearances in the Mollusca water; and it seemed to him that in proportion 

 described above could not wholly escape the as these bodies escaped, the oscillatory motion 

 notice of naturalists and microscopic observers, relaxed in intensity. From these facts he con- 

 Thus wefindAnt.de Heide,* a Dutch physician eluded that the motion apparent on the surface 

 of the end of the seventeenth century, observing of the part was produced by the agitation of 

 the appearance produced by the ciliary motion these vesicles or animated molecules within 

 in the Sea-mussel; he names it " motus radio- the furrows ; that is, he supposed the furrows 

 sus," or " tremulus." He found it in most parts to be covered by a membrane to which an 

 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 E P ist - *> in PP- !: P\t??' 48 ^ A a V 



he, because it proceeds from the whole sur- %**: fe^ SppfSi. "' 



t Of Microscopes, &c. vol. i p. 128. 



* Anat. Mytuli, &c. I2rao. Amst. 1684. f P. 214, seq. 



