OBJECT AND LINE OF ARGUMENT

THE Apodidæ have been known and studied for the last one hundred and fifty years. They have always attracted considerable attention, not only on account of their great size in comparison with other fresh-water Entomostraca, but also on account of their strange and sudden appearance in pools and ditches which owe their water entirely to the rainfall. This also is not all their morphology has been a perpetual puzzle to zoologists, and they have been classed by some with archaic forms such as the Trilobites and Limulus, while by others they have been considered as highly specialised recent forms.

This essay claims by a new explanation of the morphology of the Crustacea, to set this latter point

at rest, and to show that Apus must not only be ranked by the side of the Trilobites as one of the primitive Crustacean forms, but that it is itself a true link between the living Crustacea and the Annelida. By careful examination of the organisation of Apus, and a comparison of it with that of a carnivorous Annelid, it is possible to show, as will be done in the following pages, that Apus is perhaps the most perfect "missing link" which zoology so far possesses, perfect, not only because its morphology is easily deducible from that of a carnivorous Annelid, but also because the mechanical causes of the transformation are apparent. The Apodide will in fact be found to afford us the first complete illustration of the rise of one large animal class out of another by the simple and natural adaptation on the part of one single species of the latter to a new manner of life. Close investigation shows the Apodidae to be both morphologically and biologically an almost ideal transition form.

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More or less satisfactory transition forms between most of the great animal classes are known, but none has till now been discovered between the Annelida and the Crustacea. The object of this book is to satisfy this want, not by the discovery of a new animal, but by a new explanation of one long known and often described.

The established transition forms between the other classes of the animal kingdom still leave much to be desired. Between the Protozoa and Metazoa the transition forms are either claimed by botanists, or else, however probable, are somewhat hypothetical.

Between the Cœlenterata and the Platodes we have rival links. When reading the arguments in favour of the claims of those specialised Ctenophora, the Coloplana and Ctenoplana, we feel convinced ; but, on the other hand, when we study for ourselves under the microscope such a simple Rhabdocœle as Microstomum lineare, especially during its changes of shape when moving about under a cover glass, our former conviction fades away, and we see in it a specialised larval form of a Colenterate. Between the Platodes and the Annelids the gap seems small, but we cannot bridge it over until we decide whether the segmentation of the Annelids is a kind of axial strobilation, or the natural mechanical selection of internal symmetry. Between the Annelids and the Molluscs we have the claims of Solenogaster to attend to; but this animal is unfortunately so rare, that it will be long before we can hope to have any very thorough knowledge of its morphology. Between the Annelids and the Tracheata we have Peripatus; this highly interesting animal, has a special claim on our attention, as the Tracheata form with the Crustacea the great class known as the Arthropoda. We shall find that our explanation of the rise of the Crustacea supplies us also with a very probable clue as to the origin. of the Tracheata. The Echinodermata and the Tunicata hover almost entirely in the air. And, lastly, we have the giant trunk of the Vertebrata, the roots of which are being eagerly sought in different directions. The claims of Amphioxus and of the Ascidian larva are confidently put forward by the

majority of our leading zoologists, but there are difficulties not yet explained which make many restless, and lead them to search in other directions.

In this state of affairs it will be a clear gain and encouragement if we can connect the Annelida and the Crustacea in the way described in these pages, in which we show how a typical carnivorous Annelid (presumably a Nereid, though probably not so specialised as any modern member of that family) can, by a simple and natural adaptation to a new manner of life, be established as the ground type of Apus. We mean a great deal by this expression "ground type," much more than any mere general resemblance of organisation; we mean that every single organ of Apus, where it does not resemble that of its Annelid ancestor, is capable of being deduced from some organ in the latter, and, further, that the causes of the transformation are not far to seek. These are large claims; the following pages will show whether they are justifiable.

Before entering into the morphological and anatomical details upon which our deduction of the Apodidæ from a carnivorous Annelid is based, it will make the task of the reader lighter if we here set out the line of argument.

Many carnivorous Annelids have, as is well known, a protrusible pharynx, armed with teeth, which is shot out for the seizing of prey. We assume that the Annelid from which Apus is derived, adopted a habit of browsing, which rendered this protrusible

pharynx unnecessary, so that it degenerated. The Annelids afford us such a wonderful variety of forms adapted to almost every possible manner of life, that this assumption presents no difficulty. Cambrian and Silurian formations have revealed to the palæontologist abundant evidence that early Chatopods crawled about along the bottom of the seas of those times. That one of these should become specialised for feeding in the manner supposed, is not too much to ask.

The use of the pharynx just described is, as far as we can see, a clumsy method of obtaining food. The loss of it, and the adoption of a browsing method of feeding, might well be a gain. The further development of this habit would lead to a bending round of the head sufficient to enable the animal to use its anterior parapodia for pushing prey into its mouth. In time the bend of the head would become fixed, and the parapodia modified as jaws and maxillæ. The parapodia of at least a certain number of anterior trunk segments would certainly also serve to rake food together into the middle line and forward it towards the mouth. From this very simple and natural modification of a Chatopodan Annelid, we believe that all the Crustacea, living or extinct, can be deduced. To establish this, is the object of this little book, which we have called “The Apodidæ,” since it was during our study of these Phyllopods that we first caught sight of the Annelid, so effectually disguised under its Crustacean dress. Although this disguise is so complete as to have eluded all former research, yet when once under