58 



POLYPIFERA. 



to consist of a transparent sac, in which are 

 contained generally from four to six of the 

 gemmules, which, as soon as the sac is torn, 

 escape, and swim about with the greatest ac- 

 tivity, affording a most interesting subject for 

 microscopic investigation. 



When viewed with a power of 40, linear 

 measure, they are seen to be of an oval or 

 rounded form (Jig. 63, b), convex above and 



Fig. 64. 



Halodactylus diaphanus, a gemmule seen from above; 

 the cilia as when slowly acting round the margin in 

 waves. (After Farre.) 



nearly plane below, and fringed at the margin 

 with a single row of cilia, which appear to 

 vibrate in succession around the whole cir- 

 cumference. 



Under an amplification of 120 they assume 

 a different aspect (fig. 64), and their minute 

 structure is clearly discerned. Viewed as 

 opaque objects, both the body and cilia have 

 a silvery whiteness, but by transmitted light 

 the former appears of a dark brown, and the 

 cilia of a golden yellow colour. Upon the 

 most convex part of the body, which is not 

 generally in the centre, but leaning to one 

 side, are set from three to five transparent 

 bosses, surrounded by a circle, and other 

 circles are seen extending to the base of the 

 body, which is bounded by a row of promi- 

 nent tubercles. These marginal tubercles are 

 from thirty to forty in number; and from the 

 circumstance of the cilia arising from them, 

 Dr. Farre considers it probable that they are 

 for the purpose of governing their motions, 

 and therefore analogous to the muscular lobes 

 of Hydatina senta and other Rotifera figured 

 by Ehrenberg. No structure, however, could 

 be detected in these, nor in any other part of 

 the body, beyond a mere granular parenchyma. 

 When thus highly magnified, it is seen that 

 what examined with a lower power appeared 

 to be a single cilinm is, in fact, a wave of 

 cilia, and that their motion, instead of being 

 in the direction of the circumference of the 

 disc, is at right angles to it. The ciliary 

 phenomena are the most readily observed 

 when the gemmule is nearly at rest, or has 

 become languid ; it then lies either with the 

 convex or the concave side uppermost, and 

 with the cilia, which are of great length, 

 doubled in the middle upon themselves, so 

 that their extremities are brought back nearly 

 to touch the margin of the disc from which 

 they arise. The whole fringe of cilia is then 

 suddenly unfolded, and after waving up and 



down with a fanning motion, they are either 

 again folded up towards the under surface of 

 the body, or they commence their peculiar 

 action. 



As the cilia have the appearance of moving 

 in waves round the disc (fig. 64), each wave 

 may be thus analysed. From a dozen to 

 twenty cilia are concerned in the production 

 of each apparent wave, the highest point of 

 which is formed by a cilinm extended to its 

 full length, and the lowest point between 

 every two waves by one folded down com- 

 pletely upon itself, the intervening space 

 being completed b} 7 others in every degree 

 of extension, so as to present something of 

 the outline of a cone. 



As, however, the persistence of each cilium 

 in any one of these positions is only of the 

 shortest possible duration, and each takes up 

 in regular succession the action of the ad- 

 joining one, so that cilium, which by being 

 completely folded up formed the lowest be- 

 tween any two waves, now in its turn, by its 

 complete extension, forms the highest point 

 of a wave ; and thus, while the cilia are alter- 

 nately bending and unbending themselves 

 each in regular succession after the other, the 

 waves only travel onwards, whilst the cilia 

 never change their position in this direction, 

 having, in fact, no lateral motion. When the 

 waves travel very rapidly, they appear smooth 

 on one side and (ringed on the other. The 

 whole of the ciliary motions are so evidently 

 under the control of the animal, as to leave 

 no doubt on this point. The whole fringe of 

 cilia may be instantly set in motion, and as 

 instantaneously stopped, and their action regu- 

 lated to every degree of rapidity. Sometimes 

 one or two only of the waves are seen conti- 

 nuing their action, while the remainder are at 

 rest, or isolated cilia may be observed slowly 

 bending and unbending themselves, or pro- 

 jecting entirely at rest. The body is generally 

 pointed towards one extremity of the oval, 

 and at this part may be observed a bundle of 

 cilia longer than the rest, and moving very 

 rapidly. Their vibrations were in several 

 instances counted very evenly at 230 a minute, 

 continuing in action when all the others were 

 folded up. These Dr. Farre thinks may be 

 respiratory whilst the others are chiefly loco- 

 motive. Dr. Farre thinks there can be little 

 doubt that this explanation of the action of 

 the cilia in the gemmules is applicable like- 

 wise to those of the tentacula of the adult 

 animal, and not only in the Hylodactylus, but 

 throughout the class generally; for he ob- 

 served that the tentacular cilia are infinitely 

 more numerous when at rest than they appear 

 to be when in motion, and also that they vi- 

 brate, not in the direction of the plane of the 

 arms, but at right angles to it, and with the 

 same hook-like form as in the gemmules. In 

 this way the apparent travelling of the cilia 

 up one side of the arm and down the other, 

 as the eye is seduced to follow the waves 

 which they seem to produce, is at once ex- 

 plained. 



It would be impossible to account for the 



