624 



CILIA. 



the right and descending on the left of each oval, 

 as viewed from without ; but the cilia them- 

 selves are very much closer than the apparent 

 teeth, and the illusion seems to be caused by a 

 fanning motion given to them in regular and 

 quick succession, which will produce the ap- 

 pearance of waves, and each wave answers 

 here to a tooth. 



Whatever little substances alive or inanimate 

 the current of water brings, if not ejected as 

 unsuitable, lodge somewhere on the surface of 

 the branchial sac, along which each particle 

 travels horizontally with a steady slow course 

 to the front of the cavity, where it reaches a 

 downward stream of similar materials (K ) ; 

 and they proceed together, receiving accessions 

 from both sides, and enter at last, at the 

 bottom, the oesophagus (h) ; this is a small 

 flattened tube which carries them, without any 

 effort of swallowing, towards the stomach. 



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. Air. Lister, 

 p. 385, has adverted to the resemblance be- 

 tween the Ascidise 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 Ascidiae 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 laminae 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- 



* Anat. Mytuli, &c. I2ino. Ainst, 1684. 



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-water Mussel, which must have 

 been caused by the ciliary motion.-j- 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. 

 Amons 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 

 1816-17,| 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. He 

 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. He 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 



* Epist. 83, in Opp. i. p. 463, 482. Anat. et 

 Contemp. p. 52 in Opp. ii. Ibid. p. 27. Contiu. 

 Arcan. p. 17 in Opp. ii. 



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



j P. 214, srq. 



