characters, and finally ended in the existing Potamobiida and Homarina, the fossil forms left in the track of this process of evolution would be very much what they actually are. Up to the end of the Mesozoic epoch the only known Potamobiida are marine animals. And we have already seen that the facts of distribution suggest the hypothesis that they must have been so, at least up to this time.

Thus, with respect to the Etiology of the crayfishes, all the known facts are in harmony with the requirements of the hypothesis that they have been gradually evolved in the course of the Mesozoic and subsequent epochs of the world's history from a primitive Astacomorphous form.

And it is well to reflect that the only alternative supposition is, that these numerous successive and coexistent forms of insignificant animals, the differences of which require careful study for their discrimination, have been separately and independently fabricated, and put into the localities in which we find them. By whatever verbal fog the question at issue may be hidden, this is the real nature of the dilemma presented to us not only by the crayfish, but by every animal and by every plant; from man to the humblest animalcule; from the spreading beech and towering pine to the Micrococci which lie at the limit of microscopic visibility.

NOTES.

NOTE 1., CHAPTER I., p. 17.

THE CHEMICAL COMPOSITION OF THE EXOSKELETON.

THE harder parts of the exoskeleton of the crayfish contain rather more than half their weight of calcareous salts. Of these nearly seven-eighths consist of carbonate of lime, the rest being phosphate of lime.

The animal matter consists for the most part of a peculiar substance termed Chitin, which enters into the composition of the hard parts not only of the Arthropoda in general but of many other invertebrated animals. Chitin is not dissolved even by hot caustic alkalies, whence the use of solutions of caustic potash and soda in cleaning the skeletons of crayfishes. It is soluble in cold concentrated hydrochloric acid without change, and may be precipitated from its solution by the addition of water.

Chitin contains nitrogen, and according to the latest investigations (Ledderhose, "Ueber Chitin und seine Spaltungs-produkte:" Zeitschrift für Physiologische Chemie, II. 1879) its composition is represented by the formula C1 H2 N2 O10°

26

NOTE II., CHAPTER I., p. 29.

THE CRAB'S EYES, OR GASTROLITHS.

The "Gastroliths," as the "crab's eyes" may be termed, are found fully developed only in the latter part of the summer season, just before ecdysis sets in. They then give rise to rounded prominences, one on

348

NOTES.

each side of the anterior part of the cardiac division of the stomach. The proper wall of the stomach is continued over the outer surface of the prominence; and, in fact, forms the outer wall of the chamber in which the gastrolith is contained, the inner wall being formed by the cuti cular lining of the stomach. When the outer wall is cut through, it is readily detached from the convex outer surface of the gastrolith, with which it is in close contact. The inner surface of the gastrolith is usually flat or slightly concave. Sometimes it is strongly adherent to the chi tonous cuticula; but when fully formed it is readily detached fro the latter. Thus the proper wall of the stomach invests only the outer face of the gastrolith, the inner face of which is adherent to, or at any rate in close contact with, the cuticula. The gastrolith is by no means a mere concretion, but is a cuticular growth, having a definite structure. Its inner surface is smooth, but the outer surface is rough, from the projec tion of irregular ridges which form a kind of meshwork. A vertical section shows that it is composed of thin superimposed layers, of which the inner are parallel with the flat inner surface, while the outer becomes gradually concentric with the outer surface. Moreover, the inner layers are less calcified than the outer, the projections of the outer surface being particularly dense and hard. In fact, the gastroliths are very similar to other hard parts of the exoskeleton in structure, except that the densest layers are nearest the epithelial substratum, instead of furthest away from it.

When ecdysis occurs, the gastroliths are cast off along with the gastric armature in general, into the cavity of the stomach, and are there dissolved, a new cuticle being formed external to them from the proper wall of the stomach. The dissolved calcareous matter is probably used up in the formation of the new exoskeleton.

According to the observations of M. Chantran (Comptes Rendus, LXXVIII. 1874) the gastroliths begin to be formed about forty days before ecdysis takes place in crayfish of four years' old; but the interval is less in younger crayfish, and is not more than ten days during the first year after birth. When shed into the stomach during ecdysis they are ground down, not merely dissolved. of destruction and absorption takes twenty-four to thirty hours The process in very young crayfish, seventy to eighty hours in adults. the gastroliths are normally developed and re-absorbed, ecdysis is not healthily effected, and the crayfish dies in the course of the Unless process.

According to Dulk ("Chemische Untersuchung der Krebsteine:" Müller's Archiv. 1835), the gastroliths have the following composition :-

Animal matter soluble in water

Animal matter insoluble in water (probably chitin)

Phosphate of iime

Carbonate of lime

Soda reckoned as carbonate

11.43

4.33

18.60

63.16

1.41

98.93

The proportion of mineral to animal matter and of phosphate to car bonate of lime is therefore greater in the gastroliths than in the exoskeleton in general.

NOTE III., CHAPTER I., p. 31.

GROWTH OF CRAYFISH.

The statements in the text, after the words "By the end of the year," regarding the sizes of the crayfish at different ages, are given on the authority of M. Carbonnier (L'Écrevisse. Paris, 1869); but they obviously apply only to the large "Écrevisse à pieds rouges" of France, and not to the English crayfish, which appears to be identical with the "Écrevisse à pieds blancs," and is of much smaller size. According to M. Carbonnier (1. c. p. 51), the young crayfish just born is "un centimètre et demi environ," that is to say, three-fifths of an inch long. The young of the English crayfish still attached to the mother, which I have seen, rarely exceeds half this length.

M. Soubeiran ("Sur l'histoire naturelle et l'education des Écrevisses:" Comptes Rendus, LX. 1865) gives the result of his study of the growth of the crayfishes reared at Clairefontaine, near Rambouillet, in the following table:

Mean length.

Mean weight.
Grammes.

0.50

NOTE IV., CHAPTER I, p. 37.

THE ECDYSES OF CRAYFISHES.

There is a good deal of discrepancy between different observers as to the frequency of the process of ecdysis in crayfishes. In the text I have followed M. Carbonnier, but M. Chantran ("Observations sur l'histoire naturelle des Écrevisses:" Comptes Rendus, LXXI. 1870, and LXXIII. 1871), who appears to have studied the question (on the "écrevisse à pieds rouges" apparently) very carefully, declares that the young crayfish moults no fewer than eight times in the course of the first twelve months. The first moult takes place ten days after it is hatched; the second, third, fourth, and fifth, at intervals of from twenty to twenty-five days, so that the young animal moults five times in the course of the ninety to one hundred days of July, August, and September. From the latter month to the end of April in the following year, no ecdysis takes place. The sixth takes place in May, the seventh in June, and the eighth in July. In the second year of its age, the crayfish moults five times, that is to say, in August and in September, and in May, June, and July following. In the third year, the crayfish commonly moults only twice, namely in July and in September. At a greater age than this, the females moult only once a year, from August to September; while the males moult twice, first in June and July; afterwards in August and September.

The details of the process of ecdysis are discussed by Braun, “ Ueber dic histologischen Vorgänge bei der Häutung von Astacus fluviatilis.” Würzburg Arbeiten, Bd. II.

NOTE V., CHAPTER I., p. 39.

REPRODUCTION IN CRAYFISHES.

The males are said to approach the females in November, December, and January, in the case of the French crayfishes. In England they certainly begin as early as the beginning of October, if not earlier. According to M. Chantran (Comptes Rendus, 1870), and M. Gerbe (Comptes Rendus, 1858), the male seizes the female with his pincers, throws her on her back, and deposits the spermatic matter, firstly, on the external plates of the caudal fin; secondly, on the thoracic sterna around the external openings of the oviducts. During this operation, the appendages of the two first abdominal somites are carried backwards,