312 



THE POPULAE EDUCATOR. 



exist, and there are, moreover, cross relations which it is im- 

 possible to represent by such a diagram ; but the reader may 

 gather from it some idea of the nature of the relations, and 

 how impossible it is to follow them in a continuoiis descrip- 

 tion of the animal kingdom. 



Obviously, if a writer were to pursue any one of the lines 

 indicated, describing in order the animals which successively 

 come under his notice on that line, ho would be led further 

 and further from the other lines, and he must pursue his course 

 until he has arrived at the highest animal of the branch 

 which he has been ascending ; and then, like an Alpine traveller 

 who has gained the summit of a peak, he will have to look 

 around at similar elevations, between which and his own posi- 

 tion there is no stepping-stone. Thus he must, of necessity, 

 retrace his steps to the lower level, from which another ascend- 

 ing path takes its rise. 



Another course, the one we have adopted, is to break off 

 whenever a gap in the series occurs, and look around to see 

 that wo are not leaving behind us any group of animals of a 

 similar or lower grade of structure, and if we are in danger of 

 doing so, to return at once to the description of the neglected 

 group. We are the more reconciled to this method of procedure, 

 because the relations of the classes to one another are so far 

 from being determined, that each independent author has a 

 different arrangement. 



It will be seen by the diagram that, while the classes Coelen- 

 terata, Echinodermata, Annelida, and Myriapoda seem to follow 

 one another in a natural succession, leading up to the Insecta 

 that order which, of all others in the articulate sub-kingdom, 

 is perhaps the highest and most wonderfully constituted the 

 Botatoria seem to start in a rather loose relationship with the 

 Protozoa, and to lead up towards the Crustacea, a class which, 

 as represented by its higher orders, is almost as complicated in 

 structure as the Insecta, but whose lower orders are very much 

 less organised. It would seem also as though the great sub- 

 kingdom of the Mollusca is connected to the Articulates through 

 their lowest class, 'ae Polyzoa, and the class which we now have 

 to describe. 



The Botatoria were first classed with the Infusoria by Ehren- 

 berg. This classification was not to be wondered at, as all the 

 rotary animals are microscopic, and they are obtained from 

 infusions of vegetable or animal substances in water. Their 

 outward appearance is also not unlike the higher orders of the 

 Protozoa, and they move about by the same means as many of 

 these do that is, by means of the vibrations of closely-set, fine, 

 short, delicate hairs, called cilia. These cilia are so named 

 from the Latin cilium, " an eye-lash." As these are the very 

 minute organs of animals of less than i^jyth of an inch in length, 

 it may be well conceived that the name cilia has relation to the 

 form, and not to the size of the organs. The cilia in the Botatoria, 

 instead of being scattered all over the surface of the animal, as 

 in Paramecium (a Protozoon), or in the Turbellaria, are confined 

 to flat, convex lobes, situated round or near the mouth, whose 

 edges they fringe. When the animal fixes itself, the motion of 

 these lashes brings food to its mouth by causing currents of 

 water to pass towards it ; and when it relaxes its hold, then the 

 same motion causes it to progress through the water much in 

 the same way as a screw-steamer is propelled. Some of these 

 animals have the lobes all united into one circular disc, and as 

 the motion of the cilia is so ordered as to cause the appearance 

 of a number of successive waves, following one another round and 

 round the circle, it was once thought that the <disc was a kind of 

 cogged wheel whirling rapidly about a fixed axle. Hence the 

 name Botifera, or wheel-bearing animals, was given to them. 

 If this had been the right explanation of the motion, it would 

 have furnished an instance of a locomotive apparatus met with 

 nowhere else in the whole animal kingdom. A little reflection 

 concerning this contrivance led some naturalists to doubt whether 

 it really existed. Of course it is essential to the mechanical 

 device which we call a wheel that it should be entirely discon- 

 nected with the axle upon which it plays, otherwise it could 

 not revolve ; and yet it is essential that all animal structures, 

 especially to those employed in locomotive actions, that there 

 should be an organic communication between them and the 

 organs of nutrition, by means of which liquids can be sent to 

 supply the waste caused by vital actions. This liquid must also 

 be sent in such a way as not to be lost or wasted in the transit. 

 It would seem, then, that the mechanism of the wheel is incon- 



sistent with animal organism. This consideration led to a fresh 

 ptudy of the so-called wheel-animalcules. It is almost needless 

 to remark that the separate cilia were too small for their motions 

 to be distinctly traced, otherwise the mistake could never have 

 occurred. It is now supposed that the successive action of the 

 cilia gives rise to an optical illusion, by which the appearance 

 of rotation is maintained, while the organ on which the cilia is 

 situated remains stationary. This supposition is rendered almost 

 a certainty by observing the same motion in those nearly-allied 

 creatures, members of the same class, whose discs are not circular, 

 but divided into lobes. In these species it could be seen that the 

 lobes did not participate in the revolutions. The way in which 

 this optical illusion is effected will be best seen by reference 

 to the illustration (Fig. VII.). Prom this it may be seen that if 

 the cilia are deflected from the perpendicular only in one direction, 

 and that a number of these act together, so as to cross one- 

 another while the down-stroke is given, it will give rise to a. 

 number of dark points where the crossing occurs, which points, 

 by the successive action of each cilium in the series, will seem 

 to pass rapidly round the disc, while, since each returns to ita. 

 erect position separately and slowly, the eye cannot trace their 

 motion. This method of explanation is rendered more probable- 

 by the fact that these aquatic creatures are usually examined 

 under the microscope by means of transmitted light, and henca 

 anything which cuts off the rays of light at a particular point 

 will catch the eye and be followed by it. 



These cilia are found so very generally throughout the range of 

 the animal series they are placed on such different parts of ani- 

 mals, and applied to such different purposes that it is as well 

 to give some little time to the consideration of them. We have- 

 already had occasion to mention them as covering the body of 

 some Infusoria, and being applied to locomotion. They are 

 also found on the inner (as well as the outer) wall of the Coslen- 

 terata, and there cause a circulation of the fluid in the stomach. 

 They are set on the combs of the Ctenophora, or bands on the 

 larvae of the Echinodermata, and in these situations are swim- 

 ming organs. We mentioned them also as set on the tufts of 

 vessels called gills in the Annelids,, and we shall find them again 

 on the plate-like gills of Lamellibranchiata, and in these positions 

 they cause a change in the external water, and so subserve the 

 function of respiration. In the human subject they cover the 

 membrane of the nasal chambers, the trachea, and the tubes 

 leading to the lungs, and are continually employed to bring up* 

 the mucous which would else choke the passages. In all these 

 cases, and in a thousand more which might bo mentioned, their 

 action, though applied to different purposes, is essentially the 

 same. Their motion always creates an appearance of waves- 

 moving along in one definite direction, and never returning. It 

 is very easy to attribute motion to ciliary action, and, of course, 

 if the action be capable of driving liquid over the surface, it is 

 also able to move the surface upon which the cilia are set, and 

 the animal with it when that animal floats in liquid ; but it is 

 not an easy thing to explain the method of this action. When 

 we say that the circulation in sponges is maintained by the 

 ciliated chambers, the cilia of which whip the water in one 

 direction, we are repeating what a multitude of writers have 

 said before us, but we by no means explain the motion. If a 

 switch be passed violently backwards and forwards through air 

 or water, it creates a commotion, but it has no tendency tt 

 move the air or water, or the hand which holds it, in any de- 

 finite direction. How, then, do these minute switches effect 

 their purpose ? Why does not the effect of the motion in one 

 direction exactly counterbalance the effect of the motion in tha 

 other ? The writer conceives the following to be the explana- 

 tion, for which the reader will bo in some measure prepared by 

 the remarks already made on the ciliary action in the Botatoria. 

 Suppose we conceive of a number of upright rods set on a mem- 

 brane in a line corresponding to the line of the resulting waves, 

 and moving in a direction at right angles to this, or in th&- 

 direction of the waves caused by them. If one cilium or rod 

 act alone, being rapidly brought down, the liquid will be thrown 

 off from its sides to the right and left, the more obliquely in 

 proportion to the rapidity of its motion. Ib will make its way 

 by splitting the fluid, which, being thrown off laterally, will 

 finally unite behind it. But suppose the rods on each side of 

 this single rod are in motion in a parallel direction at the same 

 time, then it comes in contact, not with stagnant water, but 

 with the conjoined stream thrown off by these, which furnishes a 



