Homerythrin.-Lankester was the first to notice that the pinkish corpuscles of Sipunculus were not coloured by hæmoglobin. Schwalbe made a similar observation in the case of Phascoloma, and Krukenberg is of opinion that the pigment in the corpuscles of Phoronis is the same as in the two worms just mentioned; he gave the name hæmerythrin to the oxygenated pigment, and hæmerythrogen to the reduced pigment which has a purplish tint. The change occurring as the result of oxidation and deoxidation shows that probably this pigment is a respiratory pigment. It shows no absorption bands, and does not yield hemin crystals; but beyond that we know little or nothing about it.

Passing from the pigments to the other constituents of the blood of worms, we find very little is at present known. The cellular elements sink to the bottom of the vessel in which the blood is received, they perhaps stick to each other a little, but there is no real coagulation.

On heating the plasma a heat-coagulum forms at 64°-66°C., and filtering this off, no proteid is left in solution. This is approximately the coagulation-temperature of hæmoglobin; the same temperature causes a heat-coagulum in the blood of worms containing chlorocruorin. In those containing hæmerythrin there is in addition a second proteid coagulating at 70°C. (Krukenberg).

HÆMOCYANIN

Before proceeding to describe the blood of molluscs and arthropods, it will be here convenient to give a general description of a respiratory proteid of a blue tint, which occurs in both groups, and to which Fredericq' has given the name hæmocyanin.

The blue colour of the blood of certain snails (Helix) was noted by Erman (1817); in Astacus as well as Helix by Carus2 (1824); in Loligo, Eledone, Sepia, Cancer pagurus, and Helix pomatia by Harless 3 (1847); this observer showed that the blue colouration was the effect of exposure to the atmosphere; and he showed also the presence of copper and a trace of iron in the blood. Genth1 (1852) ascertained that the blood of Limulus assumed in a similar way a blue tint on exposure to the air, and that it contained copper, and a small amount of iron. Rabuteau and Papillon (1873) showed that in the crab and 1 Fredericq, Sur l'organisation et la physiologie du poulpe.' Extrait des Bulletins de l'Académie Royale de Belgique. 2me Série. T. xlvi. No. 11, 1878.

5

Carus, C. G., Von den äussern Lebensbedingungen der weiss- und kalt-blutigen Thiere, Leipzig, 1824, pp. 85, 86.

3 Harless, 'Ueber das blaue Blut einiger wirbellosen Thiere, und dessen Kupfergehalt,' Müller's Archiv, 1847, p. 48 et seq.

Genth, F., Ueber die Aschenbestandtheile des Blutes von Limulus Cyclops, Annalen der Chemie und Pharmacie, vol. lxxxi. 1852.

* Rabuteau and Papillon, Comptes rendus, lxxvii. 135.

Y

Octopus similar colour changes occurred. In 1874 Gorup-Besanez' added Acanthias and Unio to the above list of animals containing copper in their blood.

Jolyet and Regnard2 (1877) were the first to advance the opinion that the blue colour was united to a proteid; this was fully worked out by Fredericq3 in the following year (1878). Since then our knowledge concerning the distribution of hæmocyanin has been added to by Fredericq himself, by Ray Lankester, and by Krukenberg in numerous papers. A full list of all the animals in which it has now been described follows:-The list will be seen to consist chiefly of waterbreathing animals, but a few air-breathing animals (snails, scorpions) possess hæmocyanin also.

The properties of hæmocyanin are as follows:

a. It gives the ordinary proteid reactions.

b. It is coagulated by heat at 65°-66°C. The process of heat coagulation is however slow.

c. It is a globulin; it is incompletely precipitated in dilute solutions by acetic acid, or a stream of carbonic acid. It is also incompletely precipitated by dialysing the salt out from its solutions, or by saturation with sodium chloride. It is completely precipitated by saturation with magnesium sulphate.

d. It exists in two conditions analogous to those of hæmoglobin, viz. oxyhæmocyanin, and reduced hæmocyanin, the former having a blue colour, the latter being colourless; the blood leaving the branchial or pulmonary apparatus is blue; that in the veins is colourless. Reducing and oxidising agents produce the same changes in hæmocyanin removed from the blood.

1 Gorup-Besanez, Lehrbuch der physiologischen Chemie, Braunschweig, 1874. The blood does not, however, contain hæmocyanin.

2 Jolyet et Regnard Arch. de Physiologie, iv. 600.

3 Fredericq, loc

e. On spectroscopic examination oxyhæmocyanin shows no bands, but only a cutting off of both ends of the spectrum; on reduction the amount of shading is much diminished.

f. It always contains a small quantity of copper, which seems to take the place of the iron of hæmoglobin.

We can now pass on to the remaining invertebrate groups.

THE BLOOD OF MOLLUSCS

Lamellibranchs.-The blood of the Lamellibranchs comes rather under the heading hydrolymph than hæmolymph. It is in most cases colourless; it contains numerous colourless corpuscles. On being shed it deposits a pale, small, colourless clot. C. Schmidt' gives the following data concerning the blood of the Anodon or fresh-water mussel :

The blood of Solen and Arca contains hæmoglobin. This is present in special corpuscles, and not dissolved in the plasma as it is in most invertebrates (Lankester).

Gastropods.—These animals possess a blood much richer in solid constituents than Lamellibranchs. It is, in fact, a hæmolymph. This is well illustrated by the following numbers as estimated by Harless and v. Bibra2 in the case of the snail Helix pomatia :

Hæmoglobin is found in the blood of only one gastropod (Planorbis). The blood of most gastropods contains hæmocyanin; it was in these animals that the presence of a blue colour and of copper in the blood was first shown, though the true significance of these facts was not recognised till later. The hæmolymph of at least two gastropods, Patella and Chiton, contains no hæmocyanin, but has an orange colour. It shows no absorption bands (Krukenberg). Again, in some few cases (Doris, Tethys, Aplysia,3 and Pleurobranchus) the blood is colourless, and contains only a trace of soluble organic constituents; hence it must be called hydrolymph rather than hæmolymph (Krukenberg).

Coagulation. The only observations on the spontaneous coagulation of the shed blood (hæmolymph) are those of Krukenberg, who states that a jelly-like coagulum forms, and this rapidly becomes fluid again.

Cephalopods. Here we have a highly organised hæmolymph. It was in the blood of the octopus that Fredericq made the discovery of hæmocyanin. The blood of many other cephalopods contains the same pigment.

1 Schmidt, Lehmann's Physiol. Chem. iii. 256.

Müller's Archiv, 1847, p. 148.

3 I have had two opportunities of examining the blood of Aplysia, and I can confirm Krukenberg's statement that it is colourless and poor in organic constituents. In one case after filtering off the corpuscles, there was no proteid left in solution; in another there was merely a trace, which was precipitable by saturation with magnesium sulphate, and which, therefore, was probably of the nature of globulin. See also Cuénot, Comptes rend. cx. 724.

I take the following table from Fredericq's memoir :

With regard to the spontaneous coagulation of these fluids, there is no doubt that a clot rapidly forms when the blood is shed. This contracts somewhat in a few hours, squeezing out a small amount of serum. The only corpuscles in the blood of these animals are colourless amoeboid ones; and both Fredericq and Krukenberg lend support to Geddes's theory of a plasmodium, i.e. that the clct consists merely of adherent cells. The phenomena of coagulation as described, however, are so closely similar to those in the blood of Crustaceans and of Limulus that I am inclined to believe that in cephalopod blood we have (as in Crustacea) a ferment action converting a previously soluble fibrinogen into a substance very like fibrin.

THE BLOOD OF CRUSTACEA

The blood of a few crustaceans contains hæmoglobin dissolved in the blood plasma. These are as follows:

Daphnia (Lankester).

Cheirocephalus (Lankester).

Apus (Regnard and Blanchard).
Lernanthropus (Van Beneden).
Clavella (Van Beneden).

Cypris (Regnard and Blanchard).

Marine parasitic Crustacean (undescribed) (Van Beneden). Here the hæmoglobin is said to be contained in a special system of vessels distinct from the blood vessels.

The following is a brief summary of the chief facts ascertained with regard to the hæmolymph of the decapod crustacea.

The blood can be easily obtained by making cuts in the ventral region in the soft integuments between the abdominal segments, or in the claw. It gushes out very readily, and from a large lobster nearly half a pint can as a rule be obtained.

Colour. The blood which can be seen flowing in the ventral sinus just beneath the skin in this region appears in the vessel to be colourless. The reddish tinge which is present in some specimens when the blood is drawn is so similar to the hue of surrounding parts, that it cannot be perceived through the transparent parts of the skin. The blood when first shed is either nearly colourless, or of a reddish colour from the presence in it of a red pigment presently to be described. It

has also an opalescent or milky appearance from the presence of numerous amoeboid corpuscles. The milkiness is more marked in blood coming from the claw, than in that from the tail of the same animal. This is due to the cells being more abundant in blood from the former situation. This appearance is however but momentary, for coagulation begins to occur almost instantaneously. This is especially the case with the lobster and crayfish. In the crab coagulation is not so rapid, nor is the ultimate clot so firm and jelly-like.

The blood after being a few moments in contact with the oxygen of the atmosphere acquires an indigo-blue tinge; but the readiness with which this is seen varies in different specimens. The blue colour is due to the oxygenation of a proteid body which exists in solution in the blood plasına; in the reduced state it is colourless; in the oxidised condition it is blue. The name hæmocyanin was given to it by Fredericq.

The variation in the colour of the blood is owing to the admixture of the tint due to hæmocyanin with a varying amount of a red colouring matter. This red pigment has been noted as occurring in the crab by Jolyet and Regnard,' and in the lobster by Fredericq2; but nothing further was made out about it by these observers. This red pigment is the same as that which exists largely in the exoskeleton and in the hypoderm. It has been called there tetronerythrin, and is one of a class of pigments known as luteins or lipochromes.3 It can be dissolved out from the blood by alcohol or ether. In Astacus and the lobster the red colour as a rule predominates; but in Nephrops it is present in very small quantities.

Specific Gravity and Reaction.-The specific gravity of the blood is found to vary between 1025 and 1030. Its reaction is always faintly alkaline.

Constituents.-The blood contains the following classes of bodies: (1) Proteids.

(2) Salts. These resemble those of the water in which the animals live, being more abundant in sea-water than in fresh-water animals. The ash is also found to contain small quantities of iron and copper, the latter being combined with the proteid hæmocyanin (Fredericq).

1 Jolyet and Regnard, 'Recherches physiologiques sur la respiration des animaux aquatiques,' Archives de physiologie, iv. 600, Paris, 1877.

Fredericq, 'Note sur le sang de l'Homard,' Extrait des bulletins de l'académie de Belgique, 2me série, tome xlvii. No. 4, April, 1879.

* Merejkowski (Comptes rend. xciii. 1029) has found this pigment in 104 species of animals. He considers it may have a respiratory action like hæmoglobin on account of its distribution in the gills. It, however, is not affected by oxidising or reducing agents. Like other lipochromes it bleaches in sunlight, but this occurs equally well in a vacuum. There is no evidence that it is respiratory.