CHAPTER XVII

THE BLOOD OF INVERTEBRATE ANIMALS

INVERTEBRATE animals present in their vascular systems fluids which differ greatly from one another, and from the blood of vertebrates.

The lowest groups in the animal kingdom, the Protozoa and the Cœlentera, possess no cœlom or body cavity, and therefore no vascular system; they obtain food and oxygen direct from the water they inhabit; in the case of the Cœlentera, the water enters the enteric cavity freely. Many degenerate animals of higher groups, such as the tapeworm, have also no vascular system. Then there are other groups, such as the echinoderms (which possess the well-known water-vascular system), the acephalous molluscs (lamellibranchiata), and higher in the series the Tunicates, in which the circulating fluid is principally the sea water or fresh water in which the animal lives; but it contains dissolved in it a certain small quantity of organic substances, and in it float a number of cells like the white corpuscles of vertebrate blood. Blood of this nature may be called hydrolymph.

Lastly there are certain groups of invertebrates in which the blood is a highly organised fluid, containing in solution much organic matter, and in suspension numerous corpuscles. There is, however, in most cases no distinction between blood and lymph, and hence this variety of invertebrate blood is sometimes called hemolymph. Worms, most molluscs, and arthropods possess this variety of blood.

The hydrolymph of invertebrates discharges only one half of the functions of vertebrate blood, carrying nutriment to the tissues and organs, and removing waste products; the respiratory function of the blood is not represented, the gaseous exchanges probably occurring directly between the animal's tissues and the medium it inhabits. In hæmolymph on the other hand it is found that there is a nutritive and a respiratory function taking place. Hæmoglobin is present in the hemolymph of many animals of the invertebrate subkingdoms, and in many others it is replaced by other respiratory pigments; thus there is the pink pigment hæmerythrin, the blue pigment hæmocyanin, and the green pigment chlorocruorin. But there is this difference to be noted between the blood of vertebrates and that of invertebrates :

that whereas in the former the respiratory pigment is contained in special corpuscles (the coloured corpuscles), in the latter the pigment is dissolved in the plasma; the only corpuscles present being colourlessones. To this rule there are, however, a few exceptions; in some eight invertebrates corpuscles coloured by hæmoglobin, very like the red discs of mammals, have been found.

In the blood of certain groups of animals various other pigmentsare found (chlorophyll, tetronerythrin, &c.) which have no respiratory functions.

Many of these various forms of invertebrate blood clot when shed like vertebrate blood does. At one time the clot was considered to be merely a mass of adherent corpuscles, or plasmodium of cells (Geddes'); but it has since been shown that, in addition to the cells, there is an intercellular substance akin to fibrin which is separated from the plasma ; or at least that in many instances this is the case. It is impossible to lay down general laws concerning fluids, which differ so much as do the various forms of blood met with among the invertebrates.

It was just stated that in most invertebrate animals there is no distinction between blood and lymph. There are, however, certain exceptions to this rule: that is to say, there are cases in which the fluid in the cœlom or body cavity is. distinct from that in the vessels. The cœlom may be said to contain a fluid comparable to the lymph of vertebrates, while the vessels contain the blood. The lymph or cœlomic fluid never circulates in definite channels or lymphatic vessels as in vertebrates, though the cœlom in some cases may become subdivided into secondary spaces or sinuses.

In some cases the distinction between the two fluids is perfectly distinct; for instance, anyone may, in an earth worm, determine for himself, that a drop of the cœlomic fluid is colourless, while the blood is red. In other cases there is much dispute as to whether or no the vessels communicate with the cœlom, and hence doubt has arisen whether the blood and the cœlomic fluid are or are not identical. It is, however, possible that even if communication does exist, the two fluids might still be different from one another; for in vertebrates there is a connection between the cœlom (pleural and peritoneal cavities) and the blood vascular system ria the stomata and lymphatic vessels, and yet the lymph and the blood are distinct fluids. The following brief statement of the facts in some of the principal groups is, however, all that we have space for here:

In the Chatopods there is no doubt that blood and cœlomic fluid are distinct: thesame may be said for Phoronis, Sipunculus, and other gephyrean worms. With regard to the leeches (Hirudines) the vascular system is in undoubted communication with the cœlom; still there is at least one difference between the fluids in the two cavities: certain large corpuscles found in the sinuses of Clepsine and

1 Geddes, Proc. Roy. Soc. xxx. 252.

? A concise statement of the best ascertained facts in regard to this question will be found in a paper by A. E. Shipley, Cambridge Philosophical Soc. Proceedings, vi. 218220. This paper also enters into a somewhat similar anatomical point, viz. whether the body cavity and nephridia in certain groups open the one into the other.

Pontobdella are not found in the blood, probably because they are too large to pass through the communicating channels (Bourne).' In the nemertine worms the sinuses appear to differ in origin from those in the leeches, not being coelomic but archi-colomic (i.e. the form representing the remnants of the archical or segmentation cavity-Hubrecht), and there appears to be no connection between them and the vascular system. Another group separated a long way from these classes of worms, and in which communications exist between the vascular system and the coelom, is the Echinodermata. Hamann and Koehler showed this first in Spatangids, and Perrier and other French naturalists have shown that the same is true throughout the Echinodermata.

It will be now convenient to take up the chief invertebrate phyla, one by one, and to describe the characters of the blood as it occurs in each.

THE BLOOD OF ECHINODERMS

This is of the nature of hydrolymph, i.e. a watery fluid holding in solution saline substances (derived from the sea water) and a very small quantity of albuminous material. In it float numerous ameboid corpuscles. The following is a brief description of the varieties of corpuscles found in the perivisceral fluid of sea urchins and holothurians (Geddes 3).

1. Large amoeboid cells containing highly refracting spherules of a rich mahogany-brown colour. On exposure to the air this brown colour becomes dingy ; but in the vacuum of a mercurial air-pump it rapidly becomes normal again. There is thus considerable probability that this pigment has a respiratory function.

2. Lemon-yellow amoeboid corpuscles are also found in certain sea urchins (Arboria, Dorocidaris), but are exceedingly abundant in the perivisceral fluid of the Spatangoidea.

3. In the intestinal vessels of Spatangus amboid corpuscles, varying much in size and containing variously coloured globules (brown, yellow, purple, green, and blue), are found. The nature of these pigments has not been fully worked out, but they are probably lipo

chromes.

When a drop of the perivisceral fluid is examined microscopically, the cells at first move freely and exhibit amoeboid movements. They soon collect into irregular masses and shoot out long processes which bind the cells together.

But the clot is not a mere plasmodium; there is in addition a fibrin-like material which separates from the plasma. This contracts

1 Quart. J. Microsc. Science, xxiv. 419.

2 Ibid. xxvi. 417.

Geddes in Gamgee's Physiol. Chem. p. 134.

like fibrin after its formation, but in many of its properties it is more like mucin than fibrin (Schäfer1).

In one echinoderm of the ophiurid class, hæmoglobin has been described as occurring dissolved in the blood plasma (Fottinger2), and in another (Thyonella gemmata, a holothurian) in nucleated oval biconvex corpuscles (Howell3).

MacMunn has made numerous observations on the brown colouring matter mentioned as being observed by Geddes in the corpuscles; MacMunn made a spectroscopic examination of the perivisceral fluid of the echinoderms, Strongylocentrotus lividus, Echinus esculentus, and Sphæra. The pigment was found to vary in tint very much, brown, yellow, and red, and it darkened in the air. He named the pigment echinochrome, and considers that it has a respiratory function, the darkening being due to oxidation. In the fresh state it shows no distinct bands, but after the addition of a caustic alkali, or by solution in various solvents (water, glycerine, alcohol, ether, chloroform, &c.), it shows bands which shift in position somewhat with the solvent used; it may, however, be roughly stated that there is one wide shading covering E and another between 6 and F. The solubilities of the pigment are very remarkable; it reminds one of a lipochrome, but differs from other fatty pigments in being soluble in water. The evidence adduced as to its respiratory function does not seem to me to be absolutely conclusive.

THE BLOOD OF WORMS

The blood of worms is coloured in most cases by hæmoglobin dissolved in the plasma, in a few cases contained in special corpuscles. In other worms hæmoglobin is replaced by chlorocruorin, in others still by hæmerythrin.

The following is a list of the different worms arranged in their several classes in which these different pigments have been described :—

1 Schäfer, Proc. Roy. Soc. xxxiv. 370.

* See Lankester, Zool. Anzeiger, 1883, p. 416.

3 W. H. Howell, Studies from the Biol. Lab. Johns Hopkins Univ. Baltimore, iii. 284.

4 MacMunn, Quart. J. Microscopical Science, October 1885. Charts of the absorption spectra of echinochrome will be found with this paper.

3 The observations concerning the presence of hæmoglobin in chatopods are all by Lankester (Journ. of Anat. and Physiol. 1868, vol. ii. p. 114; Pflüger's Archiv, iv. (1871), p. 315; Proc. Roy. Soc. xxi, 71).

The worms in which hæmoglobin is present in special corpuscles are the following:-Glycera, Capitella, Phoronis, Thallasema, and Hamingia. In all other cases it is dissolved in the plasma.

Chlorocruorin.-This also is a pigment dissolved in the plasma; it is of a green colour; its decomposition products, however, indicate that it is a pigment of which hæmatin forms the basis as in hæmoglobin. It exists in two, conditions, which have been named, from the analogy to hæmoglobin and oxyhæmoglobin, chlorocruorin and oxychlorocruorin. Not only does this occur under the influence of respiratory changes in the body, but by the action of oxidising and reducing agents the same metamorphoses can be produced artificially. Oxychlorocruorin shows spectroscopically two absorption bands, one between C and D, and the other between D and E. Reduced chlorocruorin shows one band having nearly the same position as the first band just mentioned, but it is not so well defined (Lankester7).

1 Quoted by Lankester, Zool. Anzeiger, 1883, p. 416.

2 Quatrefages, see Gamgee, Phys. Chem. p. 131.

3 Vergl. Phys. Studien, 2te R. 1 Abth. p. 87.

4 Schwalbe, 'Kleinere Mittheilungen zur Histologie wirbell. Thiere,' Archiv f. mikrosk. Anat. Bonn, Bd. v. 1869, p. 248.

5 Vergl. Phys. Studien, 1ste Reihe, 3te Abth. p. 82. The name hæmerythrin is Krukenberg's.

6 Described as hemoglobin by Lankester.

Lankester, Journ. of Anat. and Physiol. vol. ii. p. 114; vol. iii. p. 119. MacMunn has also made some observations on the chlorocruorin of Sabella (Quart. J. Mic. Science, Oct. 1885). He also examined the blood of a few Serpulæ, and found that though the blood was red it gave bands somewhat like those of chlorocruorin. He considers the pigment to be one intermediate between chlorocruorin and hæmatin.