are allied. Given one of those protoplasmic bodies, of which we are unable to say certainly whether it is animal or plant, and endow it with such inherent capacities of self-modification as are manifested daily under our eyes by developing ova, and we have a sufficient reason for the existence of any plant, or of any animal.

This is the great result of comparative morphology; and it is carefully to be noted that this result is not a speculation, but a generalisation. The truths of anatomy and of embryology are generalised statements of facts of experience; the question whether an animal is more or less like another in its structure and in its development, or not, is capable of being tested by observation ; the doctrine of the unity of organisation of plants and animals is simply a mode of stating the conclusions drawn from experience. But, if it is a just mode of stating these conclusions, then it is undoubtedly conceivable that all plants and all animals may have been evolved from a common physical basis of life, by processes similar to those which we every day see at work in the evolution of individual animals and plants from that foundation.

That which is conceivable, however, is by no means. necessarily true; and no amount of purely morphological evidence can suffice to prove that the forms of life have come into existence in one way rather than another.

There is a common plan among churches, no less than

THE MORPHOLOGICAL UNITY OF LIVING THINGS. 287

among crayfishes; nevertheless the churches have certainly not been developed from a common ancestor, but have been built separately. Whether the different kinds of crayfishes have been built separately, is a problem we shall not be in a position to grapple with, until we have considered a series of facts connected with them, which have not yet been touched upon.

CHAPTER VI.

THE DISTRIBUTION AND THE ETIOLOGY OF THE

CRAYFISHES.

*

So far as I have been able to discover, all the crayfishes which inhabit the British islands agree in every point with the full description given above, at p. 230. They are abundant in some of our rivers, such as the Isis, and other affluents of the Thames; and they have been observed in those of Devon; but they appear to be absent from many others. I cannot hear of any, for example, in the Cam or the Ouse, on the east, or in the rivers of Lancashire and Cheshire, on the west. It is still more remarkable that, according to the best information I can obtain, they are absent in the Severn, though they are plentiful in the Thames and Severn canal. Dr. M'Intosh, who has paid particular attention to the fauna of Scotland, assures me that crayfish are unknown north of the Tweed. In Ireland, on the other hand, they occur in many localities; † but the question whether their diffusion, and even their introduction into this

* Moore. Magazine of Natural History. New Series, III., 1839. Thompson. Annals and Magazine of Natural History, XI., 1843.

THE NAME ASTACUS FLUVIATILIS.

289

island, has or has not been effected by artificial means, is involved in some obscurity.

English zoologists have always termed our crayfish Astacus fluviatilis; and, up to a recent period, the majority of Continental naturalists have included a corresponding form of Astacus under that specific name.

66

*

Thus M. Milne Edwards, in his classical work on the Crustacea, published in 1837, observes under the head of Ecrevisse commune. Astacus fluviatilis :" "There are two varieties of this crayfish; in the one, the rostrum gradually becomes narrower from its base onwards, and the lateral spines are situated close to its extremity; in the other, the lateral edges of the rostrum are parallel in their posterior half and the lateral spines are stronger and more remote from the end."

The "first variety," here mentioned, is known under the name of "Écrevisse à pieds blancs"† in France, by way of distinction from the "second variety," which is termed "Écrevisse à pieds rouges," on account of the more or less extensive red coloration of the forceps and ambulatory limbs. This second variety is the larger, commonly attaining five inches in length, and sometimes reaching much larger dimensions; and it is more highly esteemed for the market, on account of its better flavour.

In Germany, the two forms have long been popularly distinguished, the former by the name of "Steinkrebs,"

"Histoire Naturelle des Crustacés." † Carbonnier. ́L'Écrevisse,” p. 8.

back in the course of its development to an ovum, and that ovum gives rise to a blastoderm, from which the parts of the embryo arise in a manner essentially similar to that in which the young crayfish is developed.

Moreover, in a large proportion of the Crustacea, the embryo leaves the egg under the form of a small oval body, termed a Nauplius (fig. 73, D), provided with (usually) three pairs of appendages, which play the part of swimming limbs, and with a median eye. Changes of form accompanied by sheddings of the cuticle take place, in virtue of which the larva passes into a new stage, when it is termed a Zoca (C). In this, the three pairs of locomotive appendages of the Nauplius are metamorphosed into rudimentary antennules, antennæ, and mandibles, while two or more pairs of anterior thoracic appendages provided with exopodites and hence appearing bifurcated, subserve locomotion. The abdomen has grown out and become a notable feature of the Zoæa, but it has no appendages.

In some Podophthalmia, as in Penæus (fig. 73), the young leaves the egg as a Nauplius, and the Nauplius becomes a Zoæa. The hinder thoracic appendages, each provided with an epipodite, appear; the stalked eyes and the abdominal members are developed, and the larva passes into what is sometimes called the Mysis or Schizopod stage. The adult state differs from this chiefly in the presence of branchia and the rudimentary character of the exopodites of the five posterior thoracic limbs.