Fig. 44.—Section through eye when first forming: Hyp, hypodermis; Ln, lens; F.W.V, front wall of optic vesicle; b.w.v, back wall of vesicle; cap, capsule.
In the Chilognatha, the first Order of Myriapods, the young animal leaves the egg with three pairs of appendages; the first have already the form of antennae, the second will form the jaws, but have not yet taken their proper form, while the third pair will fuse together and alter their shape so as to form the curious plate that has already been mentioned as forming the second pair of mouth appendages. Behind the mouth appendages will come the first three pairs of legs. The whole young animal on leaving the egg is enveloped in two membranes. These membranes are secreted by the outside layer of cells in the same way that the shell or exoskeleton of the animal will be eventually formed, and represent the first two moults of the animal, which continues to moult its shell every year throughout life.
Of the Chilopoda, the second Order of Myriapods, all the families leave the egg-shell with the full number of legs, with the exception of the Lithobiidae, which have seven pairs of legs including the poison-claws. The Schizotarsia, the third Order, also have seven pairs of legs when hatched.
The legs make their appearance not one by one but in batches (in Julus terrestris in batches of five). The addition of legs and segments to the body takes place, not at the end of the body, but between the end segment and the penultimate.
This is a short sketch of the gradual development of the Myriapoda from the ovum to the fully-grown animal. It is, I am aware, a short and insufficient account of all the beautiful processes by which the different organs take their rise, but space is insufficient here, and too much detail would be out of place in a work of this nature, which only aims at giving an outline sketch of the group, which shall be intelligible to the general reader who has not made a special study of such matters. Before leaving the subject, however, I must mention a few of the points of interest which are to be learned from the examination of the course of development which has been sketched here. One of the greatest puzzles in the natural history of the Order Chilognatha has always been the double segments, as they are called; that is, in fact, the possession of two pairs of legs to each segment, which is, as we have already said, a distinguishing characteristic of the Order. As we have seen, the Chilognatha at an early stage of existence do not possess this characteristic, which is only peculiar to the adult and half-grown forms. Now what does this mean? Does each double segment in the full-grown Millepede represent two segments which have become fused together, or is each double segment, so called, a real segment resembling the segments present in the other Orders (for instance, Chilopoda), which has grown an extra pair of legs? Both these views have been advocated by distinguished naturalists. Neither of them is, in my opinion, quite right when viewed in the light cast on the subject by recent investigations into the life history of the Chilognatha.
A close examination into the minutiae of the growth of the different organs has shown us that the double characters of the double segments are more deeply seated than was imagined. The circulatory system, the nerve cord, and the first traces of segmentation in the mesoblast all show this double character, and the only single part about the segment is the broad plate covering the segment. Now in some of the most ancient of the fossil Myriapods this broad plate shows traces of a division, as if it were in reality two plates fused together. We have also to consider that the life history of the Chilognatha allows us to believe that the peculiar cylindrical shape of the body shown in the greatest degree in the Julidae is attained by the unequal development of the dorsal and ventral surfaces of the body; the ventral surface being compressed together till it is extremely narrow, and the dorsal surface, as it were, growing round it till the originally dorsal surface forms almost a complete ring round the body. Taking all this into consideration, we are justified, in my opinion, in concluding that each double segment in the Chilognatha is not two segments fused together, nor a single segment bearing two pairs of legs, but is two complete segments perfect in all particulars, but united by a large dorsal plate which was originally two plates which have been fused together, and which in most Chilognatha surrounds almost the whole of two segments in the form of a ring.
Again in the Chilopoda we see that a great distinctive feature that separates them from the Chilognatha is the character of the ventral nerve cord, the cord being double and not single, a character connected with the fact that the bases of the legs are widely separated from one another, and not closely approached to each other, as in the Chilognatha. As we before said, a more minute anatomical examination showed us that this difference was not so great as appeared at first sight, the cord showing traces of a duplication. Well, are these traces superficial, or do they represent a state of affairs more or less similar to that in the Chilopoda? Embryology helps us to answer this question also. In the early stages of the Chilognatha we find that the nerve cord has exactly the form of that in Chilopoda, showing us that the appearances in the anatomy had led us to a right conclusion, and giving us a valuable confirmation of our views. These two examples will serve to show the kind of interest which attaches to embryology.
Palaeontology.
We have seen that embryology enables us to look at the structure of the Myriapods from a new standpoint, and to correct and supplement the knowledge gained from an examination of the adult animal. In the same way a study of the forms of Myriapods which have become extinct on the globe, and have been preserved to us in a fossil form, gives a further opportunity of considering the relations of one form to another, and again of the relations of our group to other groups of animals now existing on the earth. Myriapod fossils have been found in strata of great antiquity. The oldest of such fossils must have been among the first land animals. The figure below shows a fossil Myriapod found in America, belonging to the Order of the Protosyngnatha which are only found in the Palaeozoic strata. It is a good example of the manner in which Myriapods were protected by bundles of bristles in the same way as the Polyxenus of the present time.