stood, it is found to be very superficial. We shall be able to show, in the following pages, that the Apodida agree in almost every detail of their organisation with such an Annelid, and that any disagreement is chiefly due to further specialisation in adaptation to the new manner of life described.

Commencing with the head, we shall show how the morphology of the typical Crustacean head is easily explained by the bending round of the five anterior segments of such an Annelid for the purpose of browsing.

The trunk of Apus will be shown to be a true link between the many-segmented Annelids, and the Crustacea with their small and almost constant number of segments. The rise of the shield will be briefly mentioned, a fuller account of it being reserved till we compare Apus with the Trilobites.

The gradual transformation of the Annelidan cuticle into the exoskeleton of the Crustacea, to which many of the changes in the inner organisation of the latter are to be referred, will be found well illustrated by the Apodidæ.

The Annelidan parapodia (with their dorsal and ventral branches) will be shown to be capable of developing every form of Crustacean limb, the reasons for the suppression of one part and the development of another being generally fairly evident, Apus again supplying the clue.

Coming to the inner organisation, we shall take in turn, the musculature, the nervous system, the sensory organs, the alimentary canal, the circulatory system,

the excretory and other glands, and, lastly, the reproductive organs. We shall either point out the resemblances in each case between these organs and those of our Annelid, or else show how they can be deduced from Annelidan organs. It will be found that while some of the modifications of Annelidan into Crustacean organs are easy to follow, the explanation of others has to be sought, and may thus appear to be, in some cases, far-fetched.

And here we must remind our readers that it is enough for our argument if we can show that such a deduction is possible. It is not essential to our theory that we should show exactly how the inner transformations actually took place. Our explanations may themselves be incorrect, but the validity of our argument can only be seriously weakened by showing that a set of organs in Apus could not possibly have been derived from any organs in the Annelida; or that the improbability of such a transformation is so great that no experienced morphologist would accept it.

We shall conclude the first part of this essay by an appeal to the Nauplius, to see whether it bears out our theory that Apus is the original form of the majority of the modern Crustacea; or, in other words, whether Apus can itself claim to be the proto-Nauplius of zoologists. We shall endeavour to describe the exact morphology of the Nauplius considered as the Apus larva or the Apus-stage in the development of the other Crustacea.

This will conclude Part I., which we hope will have shown that, so far as such claims can be based purely

upon morphological, anatomical, and biological reasoning, the Apodidæ deserve to take the place we assign them as an almost ideal transition form between the Annelida and the Crustacea. Here, as stated in the Preface, we thought to leave the matter as an interesting suggestion. Fortunately, however, we have the means of testing the accuracy of our conclusions...

Admitting, on the one hand, that the confirmatory evidence as to the truth of our theory given by the Nauplius need by no means be conclusive, we maintain, on the other hand, that the answer which we receive to our appeal to palæontology and to such archaic living forms as Limulus must be decisive. Thus we enter upon the second part of our essay in order to obtain a final "yea" or "nay" as to whether our theory is, as a whole, but a morphological tour de force, or a fairly close guess at the truth.

We commence with Limulus, and show that if Apus is to be derived from an Annelid with the first five segments bent round ventrally, Limulus must have had a similar origin.

In the second section we venture into the dangerous realm of the Trilobites. The mystery which surrounds these primitive Crustacea is so great, that every announcement of a new discovery bearing upon their morphology meets with more or less scepticism. Nevertheless, we believe that we can prove that our derivation of Apus from a bent Annelid reveals the Trilobites also in their true light, as so many attempts of browsing Crustacean-Annelids to adapt themselves to their surroundings-attempts which, in the long

run, proved unsuccessful, for reasons which we shall try to point out.

After briefly discussing the Eurypteridæ, we shall give an outline sketch of a new classification of the Crustacea based upon our theory, showing that while only one group of modern Crustacea admits of derivation from the Trilobites, all the rest, except Limulus, can be deduced from the Apodidæ. We shall see reasons for believing that it was the development of the shield, either as bivalve shell, or as a large fold of the tergum of the fifth segment, which led to success in the struggle for existence.

We should here say something as to the preservation of the Apodidæ through so many geological ages. This is explained by the manner of life of the animals. They usually appear in ditches and pools dependent on the rainfall. In such waters they naturally come little into competition with other animals. The dry seasons are bridged over by the eggs being preserved in the mud. In this strange but perfectly natural way, Apus has, from the earliest times, been so completely isolated that its preservation presents no difficulty. Its presence in every part of the globe, with almost always the same manner of life, is a sign of its great antiquity.

The fact that no true fossil Apodidæ are found, among the rich yield of Crustacean remains of the Silurian strata, admits of simple explanation. We say no true Apodidæ, for we shall find that such forms as Hymenocaris and Ceratiocaris, though perhaps somewhat more specialised, were probably very closely

related to the Apodidæ. Both these points will be discussed more in detail in Part II.

Finally, in a short concluding section we shall show that the method of differentiation which turned the Annelid into the Crustacean throws a flood of light on the origin of the Tracheata, and on some of the morphological differences which separate these two divisions of the Arthropoda.

Several new points in the anatomy of Apus will be described and illustrated. Where these do not bear directly upon the subject, they will be given in full in an appendix, so as not to interfere with the course of the argument.