normal specimens appeared to prefer pale red against the 8 in dark blue; of blind individuals, 536 were found in the first, and 406 in the latter colour; with colours of about equal intensity, 474 were found in the red, and 176 in the blue.

The proportion of individuals preferring a good light devoid of ultra-violet rays was as 2 to 1 of those found in darkish ultra-violet light; as between green and blue, the proportion was 3 to 1 of the respective colours for unblinded, and about 1 to 1 for blinded individuals. Thus blinded animals are shown to be sensitive to both quantitative and qualitative differences in light.

Graber considers the above facts to be in accordance with the theory of evolution of special optical organs (eyes) from generalized ones (skin); as the reactions of these hypothetical dermal organs resemble those of the former, and their inferior activity is quite natural. This agreement favours the interpretation of the phenomena as due to an inferior degree of vision, and not to the results of thermal or chemical influences acting on the animals experimented on.

B. INVERTEBRATA.

Nerve-centres of Invertebrata.* —W. Vignal has examined the nervous system of various groups of the higher invertebrates and comes to the following, among other, conclusions :

In the Crustacea the cells of the ganglia are nearly all unipolar, and almost always consist of a viscous granular substance, in which the nucleus is slightly and the nucleoli highly refractive. Bipolar and multipolar cells are also present. The nerve-fibres forming the connectives, the commissures, and the nerves have a proper wall, on the surface or in the interior of which there are oval nuclei; the inclosed substance is viscid and slightly granular, and contains a central bundle of fibrils, or the fibrils are isolated. The central nerve-chain and the nerves are invested in two sheaths, one of which is structureless, and appears to be of a cuticular nature, while the other is formed of imbricated lamellæ, which, in the macrourous crustacea, forms a partition in the connectives. The nerve-cells on the ventral face of a ganglion send off prolongations into its centre; this centre is formed of nerve-fibres, and of prolongations from the cells; the two are closely united and form a plexus whence the nerves are given off. The gastro-intestinal nerves are composed of fine fibres which have the same structure as those of the ventral chain. They form two plexuses, along which nerve-cells are to be observed.

In the Mollusca bipolar or multipolar cells are very rarely found among the cells of the ganglia, and this is especially the case in the Gasteropoda. The nerve-cells are formed of a ganglionic globe on the surface, and in the interior there are fine fibrils; among these are fine fatty granulations, which are sometimes variously coloured. The ganglionic globe, which has no investing membrane, contains a large nucleus and one or more nucleoli. The nerves and connectives are formed by fibres of very various sizes, which are separated from one

*Arch. Zool. Expér. et Gén., i. (1883) pp. 267-408 (4 pls.).

The

another by partitions developed from the sheath of the nerve. fibres themselves are made up of fibrils which are inclosed in a slightly refractive and feebly granular substance. The myenteric plexus forms, along the digestive tube, a triple plexus, on the branches of which ganglionic cells are irregularly scattered. The centre of the ganglia is formed by a fibrillar substance and a slightly refractive body, which is of the same nature as the peripheral matter of the cells; the central fibrils have no definite arrangement; the nerves arise from the centre of them. The envelope of the nervous system is formed by a lamellar connective tissue, which is composed of fine fibrils. Among the cells of the ganglia a peculiar kind of connective cell was observed; this was oval, and contained a large nucleus; from the two poles of the cell long fibrils are given off.

In the Hirudinea all the ganglionic nerve-cells are unipolar; those of the gastro-intestinal system have the same essential structure but are not invested in a proper membrane, the sheath that invests them being part of that system which has been compared by Ranvier to Henle's sheath in vertebrates. The fibres that make up the nerves vary in size, and are separated from one another by thick partitions, and are composed of fibrils inclosed in a slightly granular protoplasm. The sympathetic system forms a double plexus along the digestive tube, and on its branches are developed ganglionic cells. The connective chain is formed by three nervous cylinders; no nuclei are to be seen either in the protoplasm of the connectives or of the nerves. No multipolar nerve-cells are to be found in the centre of the ganglia, as Walter and Hermann have imagined. The investment of the nervous system is a continuous sheath which is only open near the ends of the nerves.

The last group dealt with is that of the Oligochata, and in it we find that the nerve-cells of the cerebral and ventral ganglia are mostly unipolar, and are formed of a viscous slightly granular substance. Near the homogeneous nucleus fatty granulations are to be found. Bipolar and multipolar cells are also to be observed, but they do not occupy any definite position. The nerve-fibres form the columns of the chain, have no proper walls, but are simply bounded by the partitions of connective tissue; these tubes are formed of a viscous and almost homogeneous substance, which is only feebly coloured by osmic acid; these fibres anastomose with one another.

The giant nerve-tubes are three in number, and extend along almost the whole length of the chain. The central, which is the largest, commences at the middle of the first ganglion, and the other two at the second; they end at the terminal ganglia. They appear to have no relation to the nerve-fibres.

The nerves have the same structures as the fibres of the columns. The whole system (with the exception of the cerebral ganglia) is completely invested in three sheaths-epithelial, muscular, and structureless (of a cuticular character); the first and third are alone formed on the cerebral ganglia.

All the ventral ganglia give off three nerves on either side. The first is very sharply distinguished into two halves.

Ser. 2.-VOL. IV.

D

Tracks of Terrestrial and Fresh-water Animals.*-T. M'K. Hughes describes some peculiar markings on mud, the manner of formation of which he has been able to observe, and points out how they explain away difficulties which have arisen in the interpretation of certain fossil tracks, showing that some of the characters most relied upon to prove the vegetable origin of the fossil forms, such as branching, solid section, &c., could be produced by animals.

His observations were made on certain pits in the district about Cambridge which are filled with the fine mud produced in washing out the phosphatic nodules from the Cambridge greensand. As the water gradually dries up, a surface of extremely fine calcareous mud is exposed. This deposit is often very finely laminated, and occasionally among the lamina old surfaces can be discovered, which, after having been exposed for some time to the air, had been covered up by a fresh inflow of watery mud into the pit. The author describes the character of the cracks made in the process of drying, and the results produced when these were filled up. He also describes the tracks made by various insects, indicating how these are modified by the degree of softness of the mud, and points out the differences in the tracks produced by insects with legs and elytra, and by annelids, such as earthworms. The marks made by various worms and larvæ which burrow in the mud are also described. Marks resembling those called Nereites and Myrianites are produced by a variety of animals. The groups of ice-spicules which are formed during a frosty night also leave their impress on the mud. The author expresses the opinion that Cruziana, Nereites, Crossopodia, and Palæochorda are mere tracks, not marine vegetation, as has been suggested in the case of the first, or, in the second, the impression of the actual body of ciliated worms.

Growth of Carapace of Crustacea and of Shell of Mollusca.tA notice is here given of T. Tullberg's essay on this subject, in which he states that the carapace of the lobster is formed by the subjacent cells, the outer part of which becomes directly converted into the hard covering; the striation is due to the fibres being imbedded in the fundamental substance; these fibres are formed by the cells at the time when the enveloping substance is deposited.

On the other hand, the shell of the Mollusca is, for the most part, a secretion from the cells of the mantle, but there is, in addition, a substance which in structure calls to mind the carapace of the lobster, where, too, the outer part of the cells gives rise to the shell-substance. The operculum of the whelk appears to be formed in the same way as its shell.

The researches have been carried on in too few, species to justify any general conclusions, but if we take into consideration the great resemblance which obtains between all chitinous formations, it hardly seems rash to suppose that they are all formed like the carapace of

* Abstr. Proc. Geol. Soc. Lond., 1883, No. 443, pp. 10-11.

† Arch. Zool. Expér. et Gén., i. (1883) pp. xi.-xiv.

In K. Svenska Vetens.-Akad. Handl., xix. (1882).

the lobster, while the great resemblance between the shells of Lamellibranchs and Gasteropods almost justifies the belief that their mode of formation is essentially the same.

Commensalism between a Fish and a Medusa.*-In a consignment from the Mauritius, G. Lunel found united Caranx melampygus and Crambessa palmipes. The fish stuck with the greater part of its body in the apertures which are formed by the four columns uniting the stomach with the nectocalyx, and traversed by the gastro-vascular canals. This union could not be explained by the hypothesis that the animal had sought out the other as its prey and means of nourishment. For the medusa belongs to a family which possesses no proper oral aperture, but only a series of microscopic pores, which can only take in very finely divided nourishment, and the fish had merely taken up his quarters in a natural hollow of the medusa, which was only enlarged, but in no way injured, by the long residence of the fish.

It was ascertained that the fisherman had taken the two animals together in that position; and that several years ago there had been seen on the coast, in a depth of about six inches below the surface, a fish of the same kind in conjunction with an anemone, and going in and out of it. The anemone into which the fish had entered was living, for it could be seen moving.

Lunel arrives at the conclusion that there are certain kinds of fish the fully grown individuals of which live at more or less considerable depths, whilst the young, either on account of an unknown peculiarity of their organization, or because they require a diet more congenial to their age, ascend with particular medusa to the upper regions of the sea, to find there the countless small pelagic animals on which they and their hosts are nourished. It is noticeable that the fish, in order to enter the medusa, must swim upon its side, therefore in a very abnormal position.

Symbiosis of Algæ and Animals.†-K. Brandt states that the occurrence of yellow cells has now been observed in the following groups of animals :-Radiolaria, Anthozoa, Hydrozoa, Foraminifera, Flagellata, Ciliata, Spongiæ, Ctenophora, Echinoderma, Bryozoa, Turbellaria, and Annelida. He is able to add the following to the list of species in which they have been detected:-Reniera cratera, Paralcyonium elegans, Aiptasia turgida, Echinocardium cordatum, Holothuria tubulosa (larva), Zoobothrium pellucidum, and Eunice gigantea.

Besides yellow and brown algae, others occur also in animals. Green algae have been found in numerous rhizopods and infusoria, also in fresh-water sponges, hydrozoa, and turbellaria. Marine sponges also contain blue-green alga, Oscillatories, and red and redviolet Florideæ. Engelmann's researches on animal chlorophyll show that some modification must, however, be made of the conclusion at

Fol's Recueil Zoologique Suisse,' i. (1883) pp. 65-74 (1 pl.).

+ Pflüger's Arch. f. gesammt. Physiologie, 1883, pp. 445-54. Of. this Journal, ii. (1882) pp. 241, 322.

which the author had previously arrived, that the occurrence of chlorophyll in animals is invariably due to the presence of inclosed algæ.

The yellow cells of different animals differ from one another very considerably in their structure; but all agree in possessing a chlorophyll-like pigment, a nucleus, and a starch-like product of assimilation. In almost all were found two different products of assimilation, viz. 1st, grains containing a vacuole, and therefore appearing like a ring in optical transverse section, never doubly refractive, always colourless or very pale blue, and coloured by pure iodine brown or violet, or, under certain circumstances, blue-violet; 2nd, compact granules, doubly refractive and of irregular form, of a reddish or violet colour, and not changed by treatment with iodine. The first of these is undoubtedly a substance allied to starch.

When large quantities of the green cells are carefully treated with filtered water, they usually assume the form of zoospores with two cilia at the anterior end; their pigments being still usually in the form of parietal plates, and having starch-grains in their interior.

Morphologically the yellow cells are very different from chlorophyll-bodies, and correspond to unicellular chlorophyllaceous algæ, while physiologically they behave altogether like chlorophyll-grains.

By a fresh series of experiments the author has confirmed the view previously held that the hosts or "phytozoa" make use, for their own nutrition, of the products of assimilation which the alga obtain in excess through the influence of light.

Mollusca.

Skin of Cephalopoda.*-P. Girod regards the dermis of cephalopods as being essentially formed of connective tissue, the cells of which may become the centre for the formation of reticulated tissue, connective bundles, pigment-cells, or the so-called iridocysts. We find two strata, one formed of pigment-cells which are motile chromatophores, the other of iridocysts. The former is the most interesting, and has been very extensively studied. For its further comprehension it is well to distinguish the two constituent parts of the chromatophore: the pigment-cell, which is nothing else than the central spot, filled with coloured granulations, and the radial bundles which form a complete crown around the cell. The chromatophore, thus constituted, moves in a space which may be called the peripheral space.

The pigment-cell varies in size according to the degree of contraction or expansion of the chromatophore. The basal cell of the radial bundle is rounded during contraction, elongated and flattened during expansion. The fibres which make up the bundle approach one another during contraction, and separate on expansion. The interfascicular spaces are elongated during contraction, wider and flatter during expansion. Girod denies the contractile muscular nature of the radial bundles, and regards them simply as formed of connective tissue. It is clear, therefore, that, on this view, the

*Arch. Zool. Expér. et Gén., i. (1883) pp. 225-66 (1 pl.).