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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 Arthropodain 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 C15 H26 N2 O10.
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 use to rounded prominences, one on 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 cuticular 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 chitonous cuticula; but when fully formed it is readily detached from 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 projection of irregular ridges which form a sort of meshwork. A vertical section show 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 ahout 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. The process of destruction and absorption takes twenty-four to thirty hours in very young crayfish, seventy to eighty hours in adults. Unless the gatroliths are normally developed and re-absorbed, ecdysis is not healthily effected, and the crayfish dies in the course of the process.
According to Dulk ("Chemische Untersuchung der Krebsteine:" Müller's Archiv. 1835), the gastroliths have the following composition:--Animal matter soluble in water 11.43 Animal matter insoluble in water (probably chitin) 4.33 Phosphate of lime 18.60 Carbonate of lime 63.16 Soda reckoned as carbonate 1.41 _____ 98.93
The proportion of mineral to animal matter and of phosphate to carbonate 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 Metres. Grammes. Crayfish of the year O.O25 O.50 " 1 year old O.05O 1.50 " 2 years old 0.070 3.50 " 3 years 0.090 6.50 " 4 years O.11O 17.50 " 5 years 0.125 18.50 " indeterminate 0.160 30.00 " very old 0.l90 l25.00
These observations must also apply to the "Écrevisse à pieds rouge."
NOTE IV., CHAPTER I, p.37.
THE ECDYSES OF CRAYFISHES.
There is a good deal of discrepancy hetween different observers as to the frequency of the process of ecdysis in crayfishes. In the text I have followed M. Carbonnier, but M. Chantran ("Ohservations sur l'histoire naturelle des Écrevisses:" Comptes Rendus, aLXXI. 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 die 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 abdonilual somites are carried backwards, the extremities of the posterior pair are inclosed in the groove of the anterior pair; and the end of the vas deferens becoming everted and prominent, the seminal matter is poured out, and runs slowly along the groove of the anterior appendage to its destination, where it hardens and assumes a vermicular aspect. The filaments of which it is composed are, in fact, tubular spermatophores, and consist of a tough case or sheath filled with seminal matter. The spoon-shaped extremity of the second abdominal appendage, working backwards and forwards in the groove of the anterior appendage, clears the seminal matter out of it, and prevents it from becoming choked.
After an interval which varies from ten to forty-five day; oviposition takes place. The female, resting on her back, bends the end of the abdomen forward over the hinder thoracic sterna, so that a chamber is formed into which the oviducts open. The eggs are passed into the chamber by one operation, usually during the night, and are plunged into a viscous grayish mucus with which it is filled. The spermatozoa pass out of the vermicular spermatophores, and mix with this fluid, in which the peculiarity of their form renders them readily recognisable. The spermatozoa are thus brought into close relation with the ova, but what actually becomes of them is unknown. The origin of the viscous matter which fills the abdominal chamber when the eggs are deposited in it, and the manner in which these become fixed to the abdominal limbs is discussed by Lereboullet ("Recherches sur le mode de fixation des oeufs aux faux pattes abdominaux dans les Écrevisses" Annales des Sciences Naturelles, 4e Ee. T. XIV. 1860), and by Braun (Arbeiten aus dem Zoologisch-Zootomischen Institut in Würzburg, II.).
NOTE VI., CHAPTER I, p.42.
ATTACHMENT OF THE YOUNG CRAYFISH TO THE MOTHER.
I observe that I had overlooked a passage in the Report on the award of the Prix Montyon for 1872, Comptes Rendus, LXXV. p.1341, in which M. Chantran is stated to hare ascertained that the young crayfishes fix themselves "en saisissant avec un de leurs pinces le filament qui suspend l'oeuf à une fausse patte de la mère." In the paper already cited from the Comptes Rendus for 1870, M. Chantran states that the young remain attached to the mother during ten days after hatching, that is to say, up to the first moult. Detached before this period, they die; but after the first moult, they sometimes leave the mother and return to her again, up to twenty-eight days, when they become independent. In a note appended to M. Chantran's paper, M. Robin states, that "the young are suspended to the abdomen of the mother by the intermediation of a chitinous hyaline filament, which extends from a point of the internal surface of the shell of the egg as far as the four most internal filaments of each of the lobes of the median membranous plate of the caudal appendage. The filaments exist when the embryos have not yet attained three-fourths of their development." Is this a larval coat? Rathke does not mention it and I have seen nothing of it in those recently hatched young which I have had the opportunity of examining.
NOTE VII., CHAPTER II., p.64.
THE "SALIVARY" GLANDS AND THE SO-CALLED "LIVER" OF THE CRAYFISH.Braun (Arbeiten aus dem zoologisch.zootomischen Institut in Würzburg, Bd. II. and III.) has described "salivary" glands in the walis of the oesophagus, in the metastoma, and in the first pair of maxillæ of the crayfish.
Hoppe-Seyler (Pflügers Archiv, Bd. XIV. 1877) finds that the yellow fluid ordinarily found in the stomachs of crayfishes always contains peptone. It dissolves fibrin readily, without swelling it up at ordinary temperatures; more quickly at 40 degrees Centigrade. The action is delayed by even a trace of hydrochloric acid, and is stopped by the addition of a few drops of water containing 0.2 per cent of that acid. By adding alcohol to the yellow fiuld, a precipitate is obtained, which is soluble in water and in glycerine. The aqueous solution of the precipitate has a strong digestive action on fibrin, which is arrested by acidulation with hydrochloric acid. These reactions show that the fluid is very similar to, if not identical with, the pancreatic finid of vertebrates.
The secretion of the "liver" taken directly from that gland, has a more strongly acid reaction than the fluid in the stomach, but has similar digestive properties. So has an aqueous extract of the gland, and a watery solution of the alcoholic precipitate. The aqueous extract also possesses a strong diastatic action on starch, and breaks up olive oil. There is no more glycogen in the "liver" than is to be found in other organs, and no constituents of true bile are to be met with.
NOTE VIII., CHAPTER II., p.81.
ANAL RESPIRATION IN CRAYFISH.
Lereboullet ("Note sur une respiration anale observée chez plusieurs Crustacés; " Mémoires de la Société d'Histoire Naturelle de Strasbourg, IV. 1850) has drawn attention to what he terms "anal respiration" in young crayfish, in which he observed water to be alternately taken into and expelled from the rectum fifteen to seventeen times in a minute. I have never been able to observe anything of this kind in the uninjured adult animal, but if the thoracic ganglia are destroyed, a regular rhythmical dilatation and closing of the anal end of the rectum at once sets in, and goes on as long as the hindermost ganglia of the abdomen retain their integrity. I am much disposed to imagine that the rhythmical movement is inhibited, when the uninjured crayfish is held in such a position that the vent can be examined.
NOTE IX. CHAPTER II., p.82.
THE GREEN GLAND.
The existence of guanin in the green gland rests on the authority of Will and Gorup-Besanez (Gelehrte Anzeigen, d. k. Baienzschen Akademie, No.233, 1848), who say that in this organ and in the organ of Bojanus of the freshwater mussel, they found "a substance the reactions of which with the greatest probability indicate guanin," but that they had been unable to obtain sufficient material to give decisive results.
Leydig (Lehrbuch der Histologie, p.467) long ago stated that the green gland consists of a much convoluted tube containing granular cells disposed around a central cavity. Wassillew ("Ueber die Niere des Flusskrebses:" Zoologischer Anzeiger, I. 1878) supports the same view, giving a full account of the minute structure of the organ, and comparing it with its homologues in the Copepoda and Phyllopoda.
NOTE X., CHAPTER III., p.105.
THE ANATOMY OF THE NERVOUS SYSTEM OF THE CRAYFISH.
The details respecting the origin and the distribution of the nerves are lntentionally omitted. See the memoir by Lemoine of which the title is given in the "Bibliography."
NOTE XI., CHAPTER III., p.110.
THE FUNCTIONS OF THE NERVOUS SYSTEM OF THE CRAYFISH.
Mr. J. Ward, in his "Observations on the Physiology of the Nervous System of the Crayfish," (Proceedings of the Reyal Society, 1879) has given an account of a number of interesting and important experiments on this subject.
NOTE XII., CHAPTER III. p. 124.
THE THEORY OF MOSAIC VISION.
Oscar Schmidt ("Die Form der Krystalkegel im Arthropoden Auge;" Zeitschrift für Wissenschaftliche Zoologie, XXX. 1878) has pointed out certain difficulties in the way of the universal application of the theory of mosaic vision in its present form, which are well worthy of consideration. I do not think, however, that the substance of the theory is affected by Schmidt's objections.
NOTE XIII., CHAPTER III.. p. 135.
Since the discovery of the spermatozoa of the crayfish in 1835-36 by Henle and von Siebold, the structure and development of these bodies have been repeatedly studied. The latest discussion of the subject is contained in a memoir of Dr. C. Grobben ("Beiträge zur Kenntniss der männlichen Geschlechtsorgane der Dekapoden:" Wien, 1878). There is no doubt that the spermatozoon consists of a flattened or hemispherical body, produced at its circumference into a greater or less number of long tapering curved processes (fig. 34 F). In the interior of this are two structures, one of which occupies the greater part of the body, and, when the latter lies flat, looks like a double ring. This may be called, for distinctness' sake, the annulate corpuscle. The other is a much smaller oval corpuscle, which lies on one side of the first. The annulate corpuscle is dense, and strongly refracting; the oval corpuscle is soft, and less sharply defined. Dr. Grobben describes the annulate corpuscle as "napfartig," or cup-shaped; closed below, open above, and with the upper edge turned inwards, and applied to the inner side of the wall of the cup. It appeared to me, on the other hand, that the annulate corpuscle is really a hollow ring, somewhat like one of the ring-shaped air-cushions one sees, on a very small scale. Dr. Grobben describes the spermatoblastic cells of the testis and their nuclear spindles; but his account of the development of the spermatozoa does not agree with my own observations, which, so far as they have gone, lead me to infer that the annulate corpuscle of the spermatozoon is the metamorphosed nucleus of the cell from which the spermatozoon is developed. For want of material, however, I was unable to bring my investigations to a satisfactory termination, and I speak with reserve.
NOTE XIV., CHAPTER IV., p. 174.
THE MORPHOLOGY OF THE CRAYFISH.
The founder of the morphology of the Crustacea, M. Milne Edwards, counts the telson as a somite, and consequently considers that twenty-one somites enter into the composition of the body in the Podophthalmia. Moreover, he assigns the anterior seven somites to the head, the middle seven to the thorax, and the hinder seven to the abdomen. There is a tempting aspect of symmetry about this arrangement; but as to the limits of the head, the natural line of demarcation between it and the thorax seems to me to be so clearly indicated between the somite which bears the second maxillæ and that which carries the first maxillipedes in the Crustacea, and between the homologous somites in Insects, that I have no hesitation in retainlng the grouping which I have for many years adopted. The exact nature of the telson needs to be elucidated, but I can find no ground for regarding it as the homologue of a single somite. It will be observed that these differences of opinion turn upon questions of grouping and nomenclature. It would make no diiference to the general argument if it were admitted that the whole body consists of twenty-one somites and the head of seven.
NOTE XV., CHAPTER IV., p.199.
THE HISTOLOGY OF THE CRAYFISH.
In dealing with the histology of the crayfish I have been obliged to content myself with stating the facts as they appear to me. The discussion of the interpretations put upon these facts by other observers, especially in the case of those tissues, such as muscle, on which there is as yet no complete agreement even as to matters of observation, would require a whole treatise to itself.
NOTE XVI., CHAPTER IV., p.221.
THE DEVELOPMENT OF THE CRAYFISH.
The remark made in the last note applies still more strongly to the history of the development of the crayfish. Notwithstanding the masterly memoir of Rathke, which constitutes the foundation of all our knowledge on this subject ; the subsequent investigations of Lereboullet; and the still more recent careful and exhaustive works of Reichenbach and Bobretsky, a great many points require further investigation. In all its most important features I have reason to believe that the account of the process of development given in the text, is correct.
NOTE XVII., CHAPTER VI., p. 297.
PARASITES OF CRAYFISHES.
In France and Germany crayfishes (apparently, however, only A. nobilis) are infested by parasites, belonging to the genus Branchiobdella. These are minute, flattened, vermiform animals, somewhat like small leeches, from one-half to one-third of an inch in length, which attach theneselves to the under side of the abdomen (B. parasitica), or to the gills (B. astaci), and live on the blood and on the eggs of the crayfish. A full account of this parasite, with reference to the literature of the subject, is given by Dormer ("Ueher die Gattung Branchiobdella:" Zeitschrift für Wiss. Zoologie, XV. 1865). According to Gay, a similar parasite is found on the Chilian crayfish. I have never met with it on the English crayfish. The Lobster has a somewhat similar parasite, Histriobdella. Girard, in the paper cited in the Bibliography, gives a curious account of the manner in which the little lamellibranchiate mollusk, Cyclas fontinalis, shuts the ends of the ambulatory limbs of crayfishes which inhabit the same waters, between its valves, so that the crayfish resembles a cat in walnut shells, and the pinched ends of the limbs become eroded and mutilated.
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