82 



THE POPULAR EDUCATOB. 



about, are placed. The contents of such a drawer are the 

 measure of our ignorance, and when we are particularly fresh 

 in spirit, or have much leisure, we open it with a confident 

 expectation that a patient study of its contents will lead us to a 

 further knowledge, and a truer and more complete arrangement. 

 Such a drawer is Cuvier's branch Radiata, and men who have felt 

 that Cuvier had forestalled all other anatomists in the arrange- 

 ment of the higher animals into their main divisions, have been 

 able to solace themselves by re-arranging the heterogeneous 

 number of animals for which the star-fish and sea-urchin stood 

 as the representatives in the mind of Cuvier. 



Inasmuch as we must dismiss this branch Radiata from our 

 system, and shall not be able to recur to it again, as we must 

 to the other branches, it is, perhaps, as well that we should 

 explain the character on which it was founded. Cuvier observed 

 that while some of the higher animals have their two sides alike, 

 . yet they could be split down the middle in one direction only, so 

 as to leave two exactly similar halves. Thus, if one of us were 

 divided from the crown of the head vertically downward, so 

 that the division passed through the mid-liiie of the back and 

 also of the breast, we should be divided into two like halves ; 

 but if the vertical division were made in any other direction, the 

 two halves, though they might be equal, would certainly be not 

 alike. If, on the other hand, a star-fish be placed flat on a 

 table, it may be bisected in more than one direction, and the 

 halves would be alike. Indeed, if we wanted to divide it into 

 like portions, we should naturally cut it into five or ten or more 

 Begments, beginning from the centre, and cutting outwards. The 

 organs are not paired on each side of one plane, but arranged 

 like the spokes of a wheel in diverging directions from a central 

 axis. This plan of structure was therefore considered as the 

 type of the branch Radiata, a radius meaning a line drawn from 

 the centre to the circumference of a circle. If this radial 

 arrangement of organs had been universal throughout this sub- 

 kingdom, and were found in no other, this would have formed a 

 well-marked division, but it is not so. Some of the organs of 

 higher animals have an apparent radial arrangement, as, for 

 instance, the booklets by which intestinal worms fix themselves. 

 In so-called radiate animals there is generally a two-sided 

 arrangement to be found. Thus, while the arms of the sea- 

 anemone are radial, stretching away on all sides, its mouth has 

 two lips and two corners. The common purple-tipped sea- 

 hedgehog (echinus) is in outward form a typical radiate, but its 

 near ally, the heart-urchin, is almost as two-sided as ourselves. 

 We therefore reject this sub-kingdom, and substitute others in 

 its stead, as will be seen in the sequel. 



Instead of at once enumerating the numbers of sub-kingdoms 

 of the animal kingdom, and appending to each a dry catalogue 

 of the characters upon which they are formed, it is, perhaps, 

 better to induce the reader to examine two animals belonging to 

 two different branches for himself, so that he may remark the 

 essential differences in structure which they manifest. Suppose, 

 bhen, he procure a prawn and a stickleback, or, if he aim at 

 larger specimens, more easily examined, he can obtain, as we 

 have done, a lobster and a haddock. If these be carefully 

 observed, first as to their external character, and then as to 

 their internal organs, there will be found some points of simi- 

 larity, but a great many points of difference. 



Both are elongated animals, and both can be divided by a 

 mid-vertical section into two similar halves. The outer covering 

 of the fish, though it is covered with small scales, is thin and 

 flexible. It offers but little resistance to pressure, and no firm 

 support, or fixed point, from which muscles can play upon the 

 limbs. It, moreover, manifests no tendency to division into seg- 

 ments or rings. Turning to the lobster, we find it is enclosed 

 in sc hard, inflexible armour, which is divided into segments or 

 rings, placed one behind the other. This division is well marked 

 and complete in the hinder part of the body, where there are 

 eeven hard annular pieces united by softer membrane. They 

 overlap one another above, but are separated below. The great 

 shield which covers the head and fore part of the body also 

 consists of fourteen segments, but they have all become united. 

 This thick, hard outer covering is the only solid part of the 

 animal, and therefore to this must be attached the muscles at 

 both ends ; that is, both at the fixed point of support from which 

 they pull, and also at the part of the body or limbs which they 

 are intended to move. This arrangement is carried out even to 

 the limbs, whose joints are likewise cased in separate hard 



tubes, and which are wielded from within. Further, there is a 

 manifest tendency for each segment of the body to have a pair 

 of limbs. Thus, beginning from behind, we find on the last 

 segment the limbs are not developed, but only indicated ; but 

 on the next they form the side lobes of the tail, and are the main 

 instruments by which the lobster darts rapidly backward when 

 alarmed. The next four segments have each paired limbs, con- 

 sisting of two small fringed plates set at the end of a joint, and 

 with them the lobster paddles quietly forward. Then comes a 

 segment with a pair of limbs composed of two joints, used for 

 other necessary purposes. Then under the great shield are the 

 walking limbs, all many -jointed. Two pairs with one claw are 

 preceded by two more terminated by small pincers ; then come 

 the formidable claws. Next come the foot-jaws and jaws. 

 There are six pairs of these, placed closely one over the other, 

 beneath the mouth ; they cannot be seen in the engraving. 

 Then come the pair of longer feelers, the shorter feelers, and 

 finally the jointed eye-stalks. Thus each of the twenty-one seg- 

 ments of which the lobster's integument is supposed to consist 

 has a pair of well-developed limbs, with the exception of the 

 last. 



How utterly different is the locomotive apparatus of the fish ! 

 The necessary hard parts upon which the muscles must play are 

 nowhere to be found on the outside. They are situated inter- 

 nally. Eunning through the centre of the body from snout to 

 tail is a bony column or axis. This axis consists of pieces 

 which are so closely united end to end that they support one 

 another, but are capable of a slight motion on one another, so 

 that the back-bone which they form can be bent and slightly 

 twisted. This back-bone, ending forward in the base of the 

 skull, is the main part of the hard skeleton which affords attach- 

 ment to the muscles which move the limbs. In this case tit i 

 tendency of each segment of the internal skeleton to produce 

 limbs is so little marked, that there are not more than two 

 pairs of paired limbs in all; and throughout this large sub- 

 kingdom, which includes brutes, birds, reptiles, and fish, there 

 are never more than this number found, though sometimes there 

 is but one pair, and sometimes none at all. These limbs are not 

 jointed hard tubes, pulled and moved by muscles running up the 

 inside of them, but they are supported by bony levers, while the 

 muscles act on them externally. 



Passing on to the other systems of internal organs, we find a 

 marked difference in the arrangement of the nervous, alimentary 

 (food), and blood circulatory systems, in relation to one other. 



In the lobster the nervous system consists of a double series 

 of rounded masses called ganglions, which commence with two 

 lying side by side (though partially united together) above the 

 mouth, and in connection with the eyes, antennas (feelers), etc. 

 From these two cords stretch back, one running on each side 

 the mouth or throat, to another double ganglion, and from this 

 cords pass back which unite the remaining nervous masses toge- 

 ther, all of which lie in a series along the floor of the tubular 

 cavity of tho body enclosed by the rings. Each ring has a double 

 ganglion of its own, but these are sometimes united together, as 

 in the lobster. The food canal runs from end to end through 

 the centre of the body, and at its front extremity passes through 

 the nervous tract (as we have seen), and opens on the under side 

 of the body. The heart is situated above the food canal, and 

 just under the hard covering of the back. We have, therefore, 

 the main blood system situated above the food canal in the 

 centre, and the nervous system below it ; these two latter, how- 

 ever, crossing one another and exchanging places just at the 

 front of the animal. All these structures are contained within 

 one tube, which is the hard covering of the animal. 



Contrasted with this arrangement is that of the fish. In this 

 animal the food canal occupies the same central position, but 

 the heart, instead of lying above it, lies on the under side. Tho 

 nervous system does not consist of a series of knots, but of a 

 continuous column, and it is contained not in the tube which 

 lodges the other viscera, but in another tube, formed of bony 

 arches springing from the back-bone, and which is super-imposed 

 on the other tube. The relative arrangement is best understood 

 by a reference to the illustration, where transverse sections 

 are given, supposed to be taken from the parts of the animals 

 where the lines marked a b cross the lateral views of the 

 lobster and haddock. 



The fish and the lobster, then, present two types of structure 

 which are utterly different in many fundamental points, and if 



