bundles cannot be regarded as taking any active part in the expansion of the chromatophores. Their importance lies in their being the agents for the fixation of the pigment-cells in the layers which they occupy. The contraction of the chromatophore is due rather to the elasticity of the capsule and the contraction of the basal cells; the expansion of the cell seems to be due to its protoplasm. The layer of iridocysts is formed of a series of plates formed from the primitive connective-tissue-cells; they have a central nucleus, and are made up of a number of rods. Where the iridocysts are only arranged in one layer they are more closely packed. In the developmental history of the layer of chromatophores the first point is the conversion of certain cells into pigment-cells; around these other cells become grouped; the intermediate cells then increase in number, or form fresh pigment-cells, and new "6 common cells," which, in their turn, are capable of proliferation. The pigment-cell, once constituted, grows rapidly, though the nucleus remains of the same size all through the period of growth. The limiting cells divide, and so increase in number till there are from twenty to thirty of them. The radial bundles are formed by the striation of the cells of the peripheral reticulum. Development of Gills of Cephalopods.-L. Joubin describes the gills of Sepia officinalis as commencing under the form of two small rods placed symmetrically in relation to the antero-posterior plane, and in the middle of what will become the posterior wall of the pallial cavity. The bud, which is primitively due to an outgrowth of the epithelial layer, soon elongates, becomes rounded at its tip, and attached by a wide base. The bud then flattens, and its hinder face becomes applied to the visceral mass, while the anterior is still covered by the mantle. One and then a second fold, and afterwards others, appear on the bud, and these form depressions on one surface which correspond to elevations on the other. Although these folds increase in number they do not occupy the whole face of the young gill; along its edge there remains a space, and in the anterior of these an efferent vessel is developed, and in the posterior the special branchial gland. Any one elevation may be regarded as a semicircle formed of three parallel arcs of cells. If these be fixed at their extremities, and if the arcs were to grow equally, we should soon have a large, deep, and more or less conical cul-de-sac. This is not, however, what happens. The cells of the median layer increase in number, and push forwards the epithelium of the convex surface, while that of the concave remains unaltered; the middle layer soon forms a stratum invested on either side by the convex epithelium; the cells of this stratum, which at first touched one another, soon become separate, and give rise to intermediate lacunæ and vessels. Each of the layers thus formed gives rise, in its turn, to a series of transversely disposed elevations, which soon form hollow outgrowths, this time on either side. Finally, in the adult there is a * Comptes Rendus, xcvii. (1883) pp. 1076-8. third series of outgrowths, which do not appear till the embryo is about to leave the egg. The efferent blood-vessel is early developed, occupies almost the centre of the organ, and is contained in the base of the layers and of the branchial gland. The efferent vessel is developed on the crest of the gill and on the outer edge of the layers just described; it has the same undulating course as the parts which carry it, and is, at the base of the gill, directly continuous with the auricle of its own side. Further Researches on Nudibranchs.*-R. Bergh prints an important paper, illustrated by five plates, as a supplement to his monograph of the family of which Polycera Cuvier is the typical genus. After a number of general notes on species and genera, among which is the description of Ohola, a new genus collected by the 'Challenger,' at Trapura, in the South Seas, the author considers the Dorididae in general, with their divisions and probable phylogeny. The genus Heterodoris of Verrill and Emerson is considered as probably belonging to a different family. The Doridida are separated into two very well marked groups by the possession of a single large retractile crown of gills, or of numerous retractile branchia: Cryptobranchiata and Phanerobranchiata respectively. The latter, connected with the typical Doridida through Staurodoris, diverge in two lines, of which the more ancient forms are Notodoris and Akiodoris. The former culminates in Placamophorus, with Ohola as a lateral branchlet. The latter passes through Acanthodoris, Goniodoris, &c., towards Ancula and Drepania. The phanerobranchiate, non-suctorial Doridida form the Polycerada (better Polyceratidae) of Bergh, and the suctorial forms his Goniodorididæ. A synopsis of the genera and species of these groups is given. They inhabit all seas, but are largest and most beautiful in the warmer regions. Functions of the Renal Sac of Heteropoda.t-L. Joliet was able to notice on a living Phyllirhoe that the renal sac was folded, and that it opened slowly; this movement was clearly due to the action of the cilia of the pericardiac orifice. When the sac was full its own orifice opened slowly, remained visible for some seconds, and then disappeared. In the Firolide there is a system of external muscles, by means of which the renal organ may perform a true diastole. The author addressed himself to the problem whether the water taken in this diastole entered into the pericardium, or whether, on the contrary, water passed out from that cavity. The results of his observations of living forms were to convince him that the water which bathes the renal cavity does not enter into the pericardium, and that it is the function of the renal sac to extract liquid from the blood, to expel it to the exterior, but not to draw water from without to pass it into the blood. In fine, we must agree with the teaching of Lacaze-Duthiers, that an organ whose principal function is to secrete the products of * Verh. Zool. Bot. Gesellsch. Wien, 1883. Cf. Science, ii. (1883) p. 748. + Comptes Rendus, xcvii. (1883) pp. 1078–81. excretion, cannot be well looked for in the course of currents which pass into the organism, but may be well sought for along a line of centrifugal currents. Interstitial Connective Substance of Mollusca.*-J. Brock 'finds that the interstitial connective substance of molluscs is very ordinarily found in the region of the central nervous system, and of the great nerves and vessels lining the inner surface of the cœlom, and on and between the viscera; the amount present varies greatly in different species, being, for example, richly developed around the central nervous system of Opisthobranchs, though very sparsely so in Pulmonates; while the conditions are reversed when we come to examine the viscera. The author deals in detail with Aplysia punctata, A. fasciata, A. depilans, Pleurobranchus sp., Pleurobranchæa meckeli, Helix pomatia, H. nemoralis, Limax agrestis, and Arion empiricorum. The observations of those who have studied the embryology of the Mollusca appear to make it certain that, in the later stages of their development, a large quantity of spindle-shaped or branched mesodermal cells are to be found in the cœlom; it is from them that, in all probability, the connective substance is derived. To connect the one with the other it is only necessary for a homogeneous intercellular substance to be secreted; by means of their processes the cells come into connection with one another, and so give rise to the network. Other cells increase in length and break up into fibrils, and thus the whole body becomes traversed by a connected network of nucleated bundles of fibrils, which are surrounded by a plexus of unaltered mesodermal cells. Yet other cells become altered in composition, and become filled with carbonate of lime or concretions of an indefinite character. If this be the mode of genesis of the interstitial substance the lowest conditions are to be found in the Opisthobranchs; the plasma-cells are exquisitely delicate bands in Pleurobranchæa, and large compact cells with sharp processes in Aplysia punctata. With regard to the vexed question of the cellular lining of the cœlom Broch comes to the following conclusion: the Enterocolia always have a peritoneal epithelium which is derived from the endoderm, and which, therefore, represents a true epithelium; the Pseudocoelia have either no (?) coelomic epithelium, or a true endothelium (Mollusca) which is derived from the mesoblast and has the morphological value of cells of connective substance. This character may be distinctly retained, as in Opisthobranchs and Pulmonata, or may attain to a higher degree of differentiation, and taking on the form of a true epithelium obscure its original character, as in Prosobranchs (?all) and in Cephalopods. Visual Organs in Solen.†-B. Sharp has been led to believe that Solen ensis and S. vagina, the common razor-shells, are possessed of visual organs, by observing that a number of these animals which were exposed in a large basin for sale in Naples retracted their siphons when his hand cast a shadow over them. Repeating the * Zeitschr. f. Wiss. Zool., xxxix. (1883) pp. 1–63 (4 pls.).; experiment at the Zoological Station, he became convinced that the retraction was due to the shadow, and not to a slight jar which might have been the cause. Upon examining the siphon, he found as many as fifty-five blackishbrown lines or grooves between, and at the base of, the short tentacular processes of the external edge. When a vertical section of these pigmented grooves is made, the cells of which they are composed are found to be very different from the ordinary epithelial cells of the surrounding tissue. The pigment-cells are from one-third to one-half longer than the latter, and consist of three distinct parts. The upper ninth or tenth part of each cell is perfectly transparent, and is not at all affected by the colouring matter used in making the preparation; the second part is deeply pigmented and opaque, and forms about one-half the cell; while the remainder consists of a clear mass which takes a slight tinge when coloured. This portion contains a well-defined nucleus filled with granular matter, and is probably the most active part of the cell. These retinal cells, if so they may called, resemble those of the very primitive eye of Patella. The value to the Solen of an organ which would enable it to detect the shadow of approaching objects as it lies imbedded in the sand, with the end of the siphon protruding, must be evident; and the structure of the cells described bears sufficient relation to those of the eyes in Patella, Fissurella, and Haliotis, to make it highly probable that they constitute true primitive visual organs. Arthropoda. a. Insecta. be Respiratory Centre of Insects.*-According to Dönhoff, the respiratory centre in the bee is situated in the anterior ganglia, and therefore the respiratory movements are put an end to by decapitation. Dr. O. Langendorff, from his investigations, finds that in the bee, wasp, and other insects, the respiratory movements are not destroyed by removal of the head, especially when by tearing, and not cutting it off, a great loss of blood is avoided; the respiratory movements show the same increased rapidity with a high temperature, slowing with a low temperature in the headless insect as in the uninjured insect. A number of experiments were also made upon Libellula depressa and other insects belonging to the Pseudoneuroptera, in which group the segmentation of the body is very marked in correspondence with their ancestral type; in these insects the respiratory centre is not merely not localized in the head, but each segment is a complete centre in itself, being capable of respiratory movements when entirely isolated. "A better example to illustrate the physiological metamerism of the insect body can hardly be imagined; each segment with its ganglion is a physiological unity!" The results of a great number of observations are fully stated in the paper, and several diagrams are given of tracings obtained of the respiratory movements. * Arch. f. Anat. u. Physiol., 1883, pp. 80-8. Chordotonal Sense-organs and the Hearing of Insects.*-In a long and elaborate account of this subject, including numerous fresh observations, Prof. V. Graber describes under the above new designation the rod-like terminal secretory structures of the nerves of certain parts (chiefly legs or wings) of insects. The general type of rod is distinguished as Scolopal, or pencil-like, being pointed at the proximal end; this form is always hollow, and its walls are extraordinarily refractive. In general, an insect has but one form of these rods. Two subordinate forms are distinguished: (a) Mononematic and (B) amphinematic, according as the distal end is, or is not, pointed like the proximal; in the mononematic the distal end runs out into a slender filament. Mononematic rods may be either (a) conocephalic, with conical heads (larva of Tabanus, of Tortrix sp., Orthoptera and some Formicida); (b) Apiocephalic (a Phryganidlarva), head blunter; (c) Conacocephalic, truncate-conically headed (Dytiscus, some Chironomus larvæ, &c.); (d) Cylindrocephalic, head of equal diameter throughout (a saw-fly larva). The amphinematic form of rod occurs in Corethra, larva of Syrphus, Pediculida. The fine distal process is to be regarded as the termination of the head of the rod, and not as a prolongation of the nervefibre. An essential difference between the amphinematic and mononematic rod is that, in the former, the nervous axial filament is firmly fixed or stretched within the cavity of the rod, while in the latter its distal end lies loose in the liquid which this cavity contains. In the "scolopophors," or tubular end-organs of the chordotonal nerves, the "chord" of the rod is a prolongation of an axial process of the basal ganglion-cell. The usually compound masses of these organs vary greatly in the number of units contained in them; from occurring singly in Tabanus most Chironomi, they may number upwards of 100 (tympanal organ of Acridida) or 200 (some "poriferous" organs) in the same sense-organ; where they are few in number they are commonly very intimately connected, so that in some cases the contours of the different tubes are almost invisible (Corethra, Ptychoptera, Tortrix, Syrphus, &c.). In some genera (Corethra, &c.) the chordotonal organ of each segment is fastened to the integument by a special "ligament" consisting of a thin-walled tube in continuation with the sheath of the nerve, and filled with a homogeneous and slightly granular mass, and extending in a direction opposite to that of the chordotonal organ itself. Of the general positions in which these organs occur, Graber states that the typical forms are always extended from one relatively immobile point of the integument to another; e. g. any one of the organs is wholly contained in one segment, and never invades the bands connecting the segments; they also show a tendency to have as great a length as the available space will admit of, and to maintain a relatively superficial position. They are as widely distributed among insects as the optic and tactile organs, in proof of which tables are given, deduced from observations (chiefly by Graber) made on upwards * Arch. f. Mikr. Anat., xx. (1882) pp. 506-640 (6 pls.). |