626 



CILIA. 



Such is an outline of the observations hitherto 

 made relative to the ciliary motion in the 

 bivalve Mollusca. We may now shortly con- 

 sider those which refer to the other classes of 

 these animals. 



Dr. Fleming,* in describing the cilia in 

 some species of Polypi, states that " analo- 

 gous hairs" exist on the branchiae of the Tri- 

 tonia, which may probably be considered as 

 forming part of the aerating organs. He 

 also mentions, in another place,f that these 

 branchiae " readily fall off, and, as if indepen- 

 dent, are capable of swimming about for a 

 short time in the water, by means of minute 

 hairs with which their surface is covered, and 

 which move rapidly, pushing forwards the 

 distal extremity." Gruithuisen, as formerly 

 mentioned, observed the ciliary motion, and 

 recognised its true nature in the Valvata bran- 

 chiata, a species of fresh-water snail. Also 

 IlaspailjJ having seen the phenomena pro- 

 duced by the gills of the Fresh-water Mussel, 

 was led by analogy to discover the same in the 

 Lymnaea and Paludina. Without being aware 

 of these previous researches, I observed the 

 ciliary motion in several different tribes of 

 marine Mollusca, and shewed that it prevailed 

 extensively among Mollusca generally. Mr. 

 Lister, as has been already stated, has subse- 

 quently discovered that it exists in the Ascidia; 

 and since then I have also found it in that 

 animal, though in a different species. 



9. Of the ciliary motion of the embryo of 

 Mollusca. The embryo of Mollusca exhibits, 

 while within the egg, a peculiar rotatory mo- 

 tion which belongs to the class of phenomena 

 we are here considering, and is referable to the 

 same cause. This motion has been observed 

 in the Gasteropodous and Bivalve Mollusca, 

 and may perhaps be found in others. 



Gasteropoda. Swammerdam states that in 

 examining the young of the viviparous water- 

 snail, while they were yet inclosed in the mem- 

 branes of the ovum, he observed the embryo 

 turning round in the contained fluid with con- 

 siderable rapidity, and, he adds, " in a very 

 elegant manner." He again mentions the fact 

 in another place.|| Baker observed the same 

 appearance in the ova of a fresh-water snail, 

 which appears to have been the common Lym- 

 naea. He says,H " when the eggs are about a 

 week old, the embryo snail may be discerned 

 in its true shape, turning itself very frequently 

 within the fine fluid in which it lies." These 

 brief notices of this remarkable fact by Swam- 

 merdam and Baker seem to have failed to ex- 

 cite the curiosity of succeeding naturalists, for 

 there would appear to be no account of any 

 subsequent researches on the subject till those 

 of Stiebel published in 1815,** who seems not 



* Mem. of Wern. Soc. of Edin. iv. p. 488. 



t Philosophy of Zoology, v. ii. p. 470. 



J Loc. cit. 



$ Biblia Naturae, p. 142. 



|| Op. cit. p. 179. 



51" Of Microscopes, &c. vol. ii. p. 325, 329. 



** Diss. sist. Lynmaei stagnalis anatomen, Goet- 

 ting, 1815, and Meckel's D putsches Archiv fiir die 

 Physiologic, Bd. i. p. 424. Bd. ii. p. 557. 



to have been aware that the fact had been pre- 

 viously noticed. Stiebel's observations were 

 made on the ova of the Lymnaeus stagnalis. 

 They were followed by those of Hugi* in 1823, 

 and Carus in 1824,f on the same species, to 

 which Carus afterwardsj (in 1827) added cor- 

 responding observations on the Paludina vivi- 

 para. About the same time (1827) Dr. Grant 

 extended the inquiry to salt-water Gasteropoda, 

 both naked and testaceous, and, as far as I 

 know, was the first to point out the cilia, which 

 are very conspicuous in salt-water species, as 

 the agents which cause the rotation. 



The eggs of the Lymnaeus (or Lymnaea) are 

 deposited in clusters, being imbedded in oblong 

 masses of gelatinous matter that are found ad- 

 hering to stones or water-plants. Each egg 

 consists of an oval pellucid membrane, con- 

 taining within it the yolk surrounded by a con- 

 siderable quantity of limpid fluid. The yolk 

 is at first round, without any obvious distinc- 

 tion of parts, but in the progress of develop- 

 ment it changes its figure, and is gradually 

 converted into the embryo, of which the shell 

 and several principal organs can soon be dis- 

 tinguished. From the descriptions of the au- 

 thors above mentioned, as well as from some 

 observations made by myself, it appears that 

 the embryo is at first motionless, but that as 

 soon as the distinction can be perceived be- 

 tween the anterior or cephalic extremity and 

 the rest of the animal, its rotatory motion com- 

 mences. This invariably goes on in the man- 

 ner indicated by the larger arrows (V, c) in the 

 annexed figure, the head or anterior extremity 



Fig. 307. 



Embryo of Lymrwea. 



continually receding. After a, time the rota- 

 tion is combined with a progressive motion, 

 by which the embryo, while turning on its 

 axis, moves onwards at the same time along 

 the inside of the egg, performing a circuit like 

 a planet in its orbit. The path described by 

 a point on the surface is indicated by the spiral 

 line in the figure. 



Stiebel, as well as the earlier observers men- 

 tioned, is silent as to the cause of this curious 

 phenomenon. Carus at first denominated it 

 a primitive or cosmic motion, without clearly 



* Isis, 1823, p. 213. 



t Von den aiissern Lebensbedingungen der 

 weiss-und kaltbliitigen Thiere. Leipz. 1824. 



\ Nova Acta Acad. Caes. Leon, vol. xiii. p. 763. 

 Von den aiiss. Lebensb, p. 59. 



