CILIA. 



627 



explaining what he meant by the term. Having 

 subsequently discovered that a current existed 

 in the fluid in an opposite direction to that 

 followed by the embryo, he ascribed the mo- 

 tion to an attraction and repulsion exerted by 

 the substance of the embryo on the surround- 

 ing fluid,* more especially at the region of the 

 body where the respiratory organ was afterwards 

 to be developed, and justly conceived that the 

 chief purpose served by it was to renew the 

 water on the respiring surface of the embryo. 

 The attraction and repulsion again he supposed 

 to be produced by an oscillatory motion which 

 he perceived on the surface of the embryo. 

 This oscillatory motion, although he describes 

 it as taking place in the substance of the animal, 

 seems to be nothing else than the usual undu- 

 latory play of moving cilia, such as has been 

 already described in other instances, indeed 

 he himself compares it to the undulation on 

 the arms of polypi. I have distinctly perceived 

 the cilia, though they are very small, in the 

 embryo of the small species of Lymnsea com- 

 mon in this country. It is the one represented 

 in the figure, but considerably magnified. The 

 current takes place along the whole of the sur- 

 face indicated by the small arrows, which also 

 mark its direction, being opposite to that in 

 which the embryo moves. The cilia, though 

 they probably exist over all this surface, were 

 distinctly seen only on the part inclosed be- 

 tween the dotted lines at a ; it required a dou- 

 blet of one-thirty-fifth of an inch focus to make 

 them visible. 



Appearances similar to those described were 

 discovered by Dr. Grant in the ova of Marine 

 Gasteropoda. In examining the embryos of 

 the Buccinum undatum and Purpura lapillus, 

 which are inclosed in groups within transparent 

 sacs, he was struck with a rapid and incessant 

 motion of the fluid in the sac towards the fore 

 part of the embryo, and he observed that this 

 motion was produced by cilia placed around 

 two funnel-shaped projections on the fore part 

 of the young animal, which form the borders 

 of a cavity in which he perceived a constant 

 revolution of floating particles. He also ob- 

 served these circles of cilia in the young of 

 other testaceous Mollusca, as the Trochus, 

 Nerita, &c. in which the embryo was seen re- 

 volving round its axis. He met with the same 

 appearance in the naked Gasteropoda, as the 

 Doris, Eolis, &c. The embryo of these re- 

 volves round its centre, and swims rapidly 

 forward by means of its cilia, when it escapes 

 from the ovum. My own observations on the 

 ova of the Buccinum agree generally with those 

 of Dr. Grant. The larger cilia are placed 

 round the prominent border of a cavity on the 

 fore part of the body, but the surface of the 

 foot and other neighbouring parts is also ciliated, 

 though the cilia are there much smaller. Dr. 

 Grant assigns various uses to these motions ; it 

 seems not to have occurred to him that they 

 were connected with respiration, although there 

 can be little doubt that they are principally 

 subservient to that function. 



* Nova Ada, xiii. p. 771. 



Acephala. The rotation of the embryo of 

 bivalves was discovered by Leeuwenhoek, and 

 described by him in one of his epistles, dated 

 October, 1695.* On examining the ova of a 

 species of Fresh-water Mussel with the micro- 

 scope, he observed the embryo turning slowly 

 round within the egg, like a sphere revolving 

 on its axis. This was at a time when the shell 

 could be distinctly perceived on the young 

 mussel ; he had failed in discovering the phe- 

 nomenon in some ova of the same species 

 which he had examined at an earlier period of 

 advancement^ He adds, that he was so much 

 delighted with the spectacle of the young Mus- 

 sels turning round within the egg, that he spent 

 two hours along with his daughter and his 

 draughtsman in contemplating it. Baster,J 

 who wrote in 1762, seems to have observed an 

 appearance of the same kind in the ova of the 

 Oyster, if we may judge from a reference by 

 Cavolini, for I have not been able to consult 

 the original. More recently (1827) Sir E. 

 Home and M. Bauer perceived the motion in 

 the embryo of the Fresh-water Mussel, as de- 

 scribed by Leeuwenhoek, but erroneously attri- 

 buted it to a small worm which pierces the 

 egg and preys on the young mussel, and which, 

 according to their view, by dragging on it pulls 

 it round in the manner described. Lastly, 

 Carus subjected the phenomenon to a more 

 careful investigation, in the course of his re- 

 searches on the development of the River Mus- 

 sel. || According to his observations the em- 

 bryo, at the time the motion becomes percepti- 

 ble, has acquired a flattened triangular shape 

 (fig. 308), the two halves of the shell cover its 

 two surfaces, and are united 

 together by the hinge at the 

 base of the triangle. When 

 the ovum is placed under 

 the microscope, the embryo 

 is seen moving round in a ho- 

 rizontal direction, as indica- 

 ted by the larger arrows, ap- 

 pearing as if it turned on the 

 centreof thelowermost shell. 

 When the embryo is extract- 

 ed from the egg, a current is perceived in the 

 water opposite that part where the current en- 

 ters and issues in the adult animal, (as shown 

 by the small arrow,) and Carus therefore attri- 

 butes its rotatory motion to an attraction and 

 repulsion exerted on the water by that part 

 of the embryo, which is afterwards to form 

 the respiratory organ. The attraction and re- 

 pulsion of the water he supposes to be pro- 

 duced by an oscillatory motion observable 

 in the substance of the animal at its surface, as 

 in the embryo of the snail, which motion, as we 

 have already seen, is in reality an undulatory 

 movement of minute cilia. As in the snail 

 also, he conceives the phenomenon to be con- 

 nected with respiration. For an account of his 



* Ep. 95. Cont. Arc. Nat. 1697, p. 26, 27, in 

 Op. torn. ii. 

 t Ibid. p. 20. 



Opuscula Subseciva, torn. ii. p. 146. 



Phil. Trans. 1827, p. 39. 



Nov. A eta, xvi. p. 27, sqq. 



Fig. 308. 



Embryo of Mussel. 



