and can only be ascertained by carefully studying the genesis of the animals.*

Fluid Mounting.-At the April Meeting of the Microscopical Section of the Troy (U.S.) Scientific Association, the Rev. A. B. Hervey described a method, which he had recently devised. In his study of the Algæ and Lichens he had been troubled, as others have been, by the difficulty of permanently mounting specimens while studying them, without waste of time or change of arrangements. Most of the methods of mounting either ruin such objects entirely or else require considerable time, care, and special appliances that are troublesome to a busy student, and therefore instructive specimens are lost. The objects may be transferred from water to Farrant's solution of gum and glycerine, and mounted without delay; but the structure is not well preserved, and air bubbles are likely to be obstinately present. The objects show best in distilled water, sea-water, camphor-water, &c.; and to mount them instantly and with uniform success he prepares cells of the gum and glycerine solution put on by means of the turn-table in the usual way. Having made cells of the required depth, and laid them aside until thoroughly dry, the inner half of the width of the cell is varnished on the turn-table with gold size, which is also allowed time to dry perfectly. Objects in water are arranged and covered in these cells with ease, and are ready after lying aside for a time varying from a few minutes to a few hours, to receive a coat of gold size or other varnish, the fluid that exudes from the cell in pressing down the cover-glass having dissolved enough of the gum cell to hold the cover in position. It has not been found that the cell is too much affected by the fluid; but if it should be so, the cell could be made of the usual cements, insoluble in water, and then coated with a thin layer of gum.†

Influence of Temperature on the Optical Constants of Glass.-In an article in the American Journal of Science and Arts' (April), Mr. C. S. Hastings gives the results of some investigations which he has made on this subject. "The most surprising fact," he considers, "which these results point out is that the variation in dispersive power attending variation in temperature is relatively enormously greater than that of the refractive power, a fact which has, he believes, escaped attention heretofore. It could hardly have escaped unheeded, however, did not a singular relation obtain in the coefficients. The dispersive powers of three specimens of glass (flint, sp. gr. 3·554; do. sp. gr. 3·151; crown, sp. gr. 2·482) computed in the ordinary way, are as 9:8:6 nearly, while the coefficients in question are as 9:6:5 nearly; hence if this relation holds approximately for all optical glasses, as is probable, an achromatic combination good for one temperature is good for all others within moderate limits."

Protecting Cap for Focussing under Water.-This, which was described in M. M. J.,' vol. viii. p. 44, appears to have been recently re-invented, under the name of " Dudgeon's Submersion Cap.”

* Ann. and Mag. Nat. Hist.,' ser. v. vol. ii. p. 251.

+ The American Naturalist,' vol. xii. p. 333.

The Optic Rod in the Crustacea and Annelida.-The following are the "conclusions" of M. Joannes Chatin, in his article on this subject in the Annales des Sciences Naturelles' (Zoology), 6th series, vol. vii. p. 31:

'In attempting to sum up the principal results, we see that the optic rod of the Crustacea presents general characters which are constant in the whole class, and also arrangements either special or of variable importance which differ according to the types examined. This should suffice to show the danger of the method too often followed, and according to which the observation of a few insects may furnish results capable of being immediately extended to the whole of the Arthropoda.

Limited externally by the cornea,' terminating internally in the ganglion of the optic nerve, the rod presents two very distinct parts, of which the characters as well as the importance differ notably-the one, internal and more or less slender, deserves more especially the name of rod; the other, external, short and swollen, but of variable shape, is the cone.

It is needless to recall here the general characters of the latter, and the signification of the central line in which it has been attempted to show the analogue of the filament of Ritter; but as far as regards the rod, I insist particularly on the value which it is proper to attribute to its transversal striæ, which do not in any way indicate a contractile tunic, but are proper to the rod which may be separated into a certain number of disks thus marked out. This disposition establishes a close relationship between the optic rod of the Articulata, and the rod of the Vertebrata.*

Such is, in short, the structure of the rod in the generality of the class; if we go back to the different types studied, the principal forms which it there presents can easily be recalled. In Astacus, Squilla, Pagurus, Eupagurus, and Paguristes, rods are met with, whose constitution is really higher, as many details show. The Cypridina offer analogous dispositions, but seem, however, to tend towards a close histological simplification; this is particularly marked in Typton, and more clearly still in Lysianassa, where the rod shows no transversal striæ and the cellules of Semper are represented only by a dark spot, from an early period of development.

Notopterophorus and Caprella scarcely differ from the types last studied, but as much cannot be said of Epimeria, in which the organic degradation is marked in a considerable degree, and leads to extremely simple forms which in Lichomolgus become still more rudimentary.

This rapid outline reminds us of the manner in which the study of the Crustacea has led us progressively to more and more simple bacillary elements. Moreover, and without wishing to enter here into the discussion of the theories to which I allude, we know the important part which many contemporary zoologists accord to the existing too heterogeneous series of Worms, whose ensemble would constitute a kind of groupe de depart' allied by a close affinity to *It is known that the researches of Boll have recently confirmed my own observations.

other branches. This opinion seems especially defensible when we examine the visual organ, which may, in these species, assume very distinct forms, and some of which recall the eyes of the Mollusca or of the Vertebrata, whilst others may be compared to the optic point of lower animals. These considerations have naturally led me to investigate whether in this group of Worms some types might not be met with, possessing rods analogous to those of the Crustacea.

The results which are found to justify this hypothesis are known. Among the Vermilia we have found eyes exactly comparable to those of the Lichomolgus, and reduced to two elements so similar to the rod of the Crustacea, that the same name cannot be withheld from them. Among Protula, Psygmobranchus, &c., one alone of these bodies suffices to constitute the organ, whilst the study of Dasychone recalls a more elevated form, that of Epimeria, for example.

Often among the Crustacea, as I have mentioned with reference to the Lichomolgus, &c., the rods may originate from a common pigmentiferous base. But what is such an arrangement if not the exact representation of that presented to us by various worms (Protula intestinum, Vermilia clavigera, &c.)? The analogies go on multiplying in this way as one advances in the study, and thus show in the strongest manner the close relationship which exists between the optical elements of these diverse animals.

Such are the principal results of my researches; these, however, must not be regarded as forming a complete history of the optic rod, to the study of which I have only attempted to bring some new facts. I hope soon to be able to complete them by a new series of observations and experiments instituted with the view to the study of the development of the rod, and to determine what characters and what relations it may present in the different ocular forms."

The Minute Structure of Stromatopora and its Allies.-Professor H. Alleyne Nicholson and Dr. J. Murie have made a lengthy communication to the Linnean Society on the above subject. Stromatopora they point out, even at the present moment, occupies a most unsettled and uncertain position, while hints and doubts flow freely as to whether it be allied to the Calcareous or the Siliceous Sponges, to the Foraminifera, to the Corals, to the Hydrozoa, or to the Polyzoa, or whether it may not be a heterogeneous assemblage of dissimilar forms, or perhaps the representative of a special and now extinct group of organisms. Unfortunately, the animal itself cannot be appealed to as affording evidence towards the solution of this problem, the remains of its habitation, or its skeletal structures alone offering data upon which any judgment on this disputed point may be arrived at. Their object is to present the results of a careful examination of a large number of specimens and sections of different forms of Stromatopora and of related groups. These results, it is hoped, will serve to throw some light upon the anatomy and systematic position of Stromatoporoids, though, as a matter of course, some points have necessarily been left doubtful or unsettled to a large extent, owing to the impossibility of obtaining access to many of the original specimens described by earlier observers.

Appended to the paper is the following summary of the author's conclusions:-"In this communication we have first given an epitome of the very diverse views held regarding Stromatopora up to the present time. We then treat of its fossil state, and show that, although the remains have been preserved in several mineral conditions, nevertheless the skeletal organization originally has been solely of a calcareous nature. We further contribute data bearing on the structural peculiarities, not only exteriorly and general, but as elucidated by microscopic research. It results that neither are the horizontal laminæ always porous, nor the vertical pillars usually tubular, as some have asserted. In one peculiar aberrant form, Cannopora, there are, in addition, large thick walled tubes penetrating the mass vertically, and undoubtedly belonging to the organism itself. In some forms, notably the genus Stromatocerium, there is a system of more or less perpendicular canals and lacunæ without walls; in others there is a paucity or even absence of such, though, in most, smaller and larger apertures open superficially. A further system of stellate obliquely-disposed canals exists, in many forms, both deeply and on the surface of the outer layers. While the typical Stromatopora are characterized by horizontal laminæ, supported by short upright pillars enclosing cuboid chambers or cells, some take on a vesicular character (Clathrodictyon), and others (Pachystroma) are destitute of pillars. Still other examples, essentially Stromatoporoid in aspect, &c., assume a more indefinite minute structure, with a tendency to a reticulate or trabecular formation. In certain forms (notably Stylodictyon) a columnar character obtains, the chambers showing a concentric arrangement round a dense but reticulate centre. Thus by their intimate structural peculiarities we attempt a tentative classification, wherein we can distinguish at least seven types of construction, which we rank provisionally as genera, and we describe en passant a few new and remarkable species.

In discussing the affinities of Stromatopora and its allies, we bring forward such evidence and argument as we believe is sufficient to warrant our excluding them in the meanwhile from alliance with the Nullipores, the Foraminifera, the Hexactinellid Sponges, the Polyzoa, the Corals, and certain fossil forms of uncertain affinities. As respects their Hydrozoal connection, we express ourselves with greater reticence, inasmuch as in both Hydractinia and Millepora not only are there certain superficial resemblances of considerable importance, but through the curious divergent form Cannopora structural peculiarities present themselves which possibly point to Hydrozoal relationships. Moreover, Mr. Carter's late very shrewd observations among the chitinous and calcareous Hydractinia necessarily render the object at issue open to further research before the decided negative can be affirmed. Mr. Moseley's* able investigations on the Hydrocorallinæ during the Challenger' Expedition, while they yield valuable hints, do not yet afford all that is desirable to unravel the knotty point. It is possible, though, that his future investigations of the ample material brought home may supply facts bearing more * 'Phil. Trans.,' 1876, vol. clxvi. pp. 91-129, pls. 8 and 9.

directly on the skeletal structure of the fossil Stromatoporoids. Lastly, respecting Sponge alliance, we are beset by obstacles, for neither do the Horny, Siliceous, nor Calcareous divisions, recent or fossil, so far as present knowledge extends, supply us with stable data whereon to assert identity. By reason of the nature of the skeletal basis the two former groups are necessarily excluded; while total absence of spicules in the Stromatopora, as widely understood, renders it impossible to class them unconditionally with the Calcareous order of the Sponges. But seeing that Hydrozoal construction, with its tubular zooidal cavities, tabulæ, &c., has not been shown to exist in the typical forms of the Stromatoporoids, and that neither in Millepora nor Hydractinia, &c., so far as we are aware, does such a system of intercommunicating passages and occasionally lacunæ without walls obtain, as exemplified in Stromatocerium, &c., we are constrained to adopt the parallel of the Siliceous Sponges with fused and adnate spicules, and assume the existence in times past of a Calcareous group of the class Spongida with a continuous skeleton composed of non-spicular granular calcareous matter. We are, however, by no means prejudiced, but hold ourselves open to conviction; for if hereafter it be demonstrated that the canal systems, &c., of the Stromatopora are not normal productions, as we at present believe them to be beyond any reasonable doubt, but branching canals bored by some low vegetable organism,' as Moseley (l. c. p. 116) avers is the case in Millepora and Pocillopora, &c., and, furthermore, that other structural Stromatoporoid peculiarities are present in undoubted members of the Hydrozoa, then we shall be willing to admit their alliance with the latter, though certainly they are aberrant types. With our present imperfect knowledge, and taking into account all the data for and against, we must at present regard them as a group per se, or, as we think justifiable on the positive and negative evidence, a new section of the Calcareous Sponges, for which we propose the term Stromatoporoidea.".

6

The Compound Microscope applied to the Examination of Electric Discharge in Gases.-In 'Nature' for September 19th † is figured and described the microscope devised by Drs. Warren De La Rue and Hugo W. Müller for this purpose. The body of the microscope is composed of two tubes (that next to the eye made of ebonite, for protection against accidental shocks) bent at an angle for convenience of observation, and at the angle is placed a revolving mirror which can be rotated by a multiplying wheel so as to make 1000 revolutions in a minute. The microscope is furnished with special mechanism to adjust the focus and the field of view.

Marine Excursion of the Birmingham Microscopical Society.-The second marine excursion took place on 19th to 27th July last, when twenty-eight members went to the island of Arran. A small steam yacht was chartered for the week; and as the result of the dredgings a beautiful series of specimens was taken, including Luidia fragillisima *The Journal of the Linnean Soc.,' vol. xiv. (Zool.), p. 187. Nature,' vol. xviii. p. 548.