Hougue; I have recently at Wimereux met with the same species of Annelid, which lives in the adult state under very peculiar conditions: it is in reality one of the most interesting forms for the genealogical classification of the Chatopoda. If we examine attentively a corm of Laomedea gelatinosa, we frequently find on the branches of this Hydrozoon, small transparent upright tubes, which may easily escape notice, so exactly do they imitate the gonothecæ of the Campanularia. Each of these tubes is inhabited by a pretty transparent Annelid, which only differs from the assumed embryo of Terebella conchilega * in having the seven tentacles practically equal to each other; at least the median one exceeds the six lateral ones but very little in length. The presence of generative products in a good many individuals assures us that these Annelids are adult. The existence of voluminous otocysts, exactly similar to those of the Molluscs, the peculiar form and disposition of the tori uncinigeri at the extremity of the ventral cirrhi of the posterior part of the body, enable us to class this Annelid in a new genus, much farther removed from Terebella than has been hitherto supposed, and presenting affinities with several families of Polychetes. I dedicate this genus to my pupil Adolphus Wartel, who was the first to find the Annelid. I call the species Wartelia gonotheca to recall the curious mimetic character which I have noticed above. The disposition of the tube of Wartelia gives it also a certain resemblance to the tubicolous Rotifers.

After the preceding facts, a retrogressive metamorphosis and a transformation so complete as Claparède had thought it, is out of the question for Terebella The embryogeny of Terebella conchilega ought to be entirely re-examined; the most complete observations which we possess at the present day on the development of Annelida of the genus Terebella are those of Milne-Edwards relating to Terebella nebulosa, Montagu.

Near to the Wartelia should probably be classed a tubicolous Annelid from the Mediterranean, described by Wilhelm Busch, † as well as the genus Lumara of Stimpson. Perhaps even the larva figured by A. Agassiz § as the embryo of Terebella fulgida, Agassiz, may also be only an embryo of a form nearly approaching Wartelia; this is what may be supposed from the general aspect of the animal and the presence of highly developed auditory capsules. It is known, indeed, that this auditory apparatus only exists in a very small number of Annelids, in other respects far removed from those which form the subject of this note.||

Hyalodiscus subtilis (Bailey).-Mr. F. Kitton, Hon. F.R.M.S., sends. * Pl. ix. fig. 6, of Claparède.

Beobachtungen über Anatomie und Entwickelung einiger wirbellosen Seethiere.' Von Dr. W. Busch. Berlin, 1851.

Stimpson (W.), 'Marine Invertebrates of Grand Manan,' 1853, p. 30. I have not been able to procure this work, which I quote from an extract of A. Agassiz.

§"On the Young Stages of a few Annelids" (Annals Lyceum Nat. Hist. of New York,' vol. viii., June, 1866), pp. 320, 321, pl. vii., figs. 19 and 19a.

M. Alf. Giard, in Comptes Rendus,' vol. lxxxvi. p. 114.

the following note:-In my remarks on this form in the previous number (p. 239), I find I am mistaken in supposing that there were no objectives of sufficient defining power to resolve the Californian Hyalodiscus (H. Franklinii) when Professor Bailey published his species. In his Notes on some new Test Objects' he mentions objectives made by Spencer, and Powell and Lealand, capable of resolving tests as difficult as the markings on H. Franklinii.

Schmidt's Atlas der Diatomaceen-Kunde.—Mr. Kitton also kindly contributes the following:-The long delay in the publication of parts 15 and 16 has arisen from the continued illness of the author. We are glad to say that he is now recovering, and he hopes to be able to publish the future parts with greater regularity.

The parts just received are devoted to that very puzzling genus, Coscinodiscus, of which 218 figures are given. The majority of them are excellent delineations, and can be easily recognized.

6

We think that too many figures of one species are given, the differences being much too slight even to constitute varieties; for example, C. subtilis, which differs but little from C. fasciculatus (by the way, the Rev. E. O'Meara describes and figures a species under this name in the Q. M. J.,' vol. vii, N. S., p. 249, pl. vii. f. 1, but it is not the same as Schmidt's species. He afterwards, in his 'Irish Diatoms,' refers it to C. Normanii of Greville, which does resemble Schmidt's C. fasciculatus, but which certainly does not resemble O'Meara's). C. curvatulus is probably only a variety of C. subtilis. Greville's C. symmetricus is simply distinguished by its larger granules. C. extravagans must be referred to Aulacodiscus if the generic characters of that genus are of any distinctive value. Judging from the figure, we should be disposed to think it an immature valve of A. Oregonus. Fig. 34 (unnamed) is no doubt also a member of that genus, as the author surmises. C. cocconeiformis is a very doubtful Coscinodiscus; it approaches very near to Cocconeis superba, Janisch, the principal difference being its circular outline, a distinction of little value. C. denarius closely resembles C. Barbadensis of Greville.*

Part 16 contains many admirable figures of the large-celled Coscinodisci; many, however, appear to us to have no claim to be considered distinct species or even varieties. C. bulliens is the form usually considered by English diatomists to be the true C. heteroporus of Ehrenberg.

In this part we have 67 figures, of which only 18 are named. C. robustus, Greville, is probably the same as C. marginatus, Ehr.

We have in previous notices of this work expressed our regret that so many figures are given to illustrate differences which are of no specific value. As our knowledge of these forms increases, we see that characters that were once considered to be good specific or generic distinctions, are now valueless. We have Triceratia that are circular, Coscinodisci with three and four sides, and Pleurosigmas perfectly straight.

* Trans. Mic. Soc.,' vol. ix. p. 45.

the following genera :

The Atlas up to the present date contains 2230 figures illustrating

In the above table we have compared the number of figures in cach genus with the number of species in Habirshaw's Catalogue. It will be seen that in most cases the figures are much in excess; but on comparing the names in the descriptions of the plates, we find the actual number of species figured is much below that in the Catalogue; for example, in the genus Actinoptychus we have 41 figures representing 23 species; Aulacodiscus, 64 figures, 24 species. A large proportion of the generic (330) and specific (6186) names in the Catalogue are only synonyms; but estimating the actual number of genera at 150, if the work is continued on its present scale, it will require 200 parts to complete it, and its publication will extend to fifty years.

Application of Freezing Methods to the Microscopic Examination of the Brain. Mr. Bevan Lewis, F.R.M.S., contributes the following (with additional matter) to Brain':* "Our handbocks teem with descriptions of the most approved methods of preparation by hardening reagents, but I cannot recall to mind any description of the method to be adopted for examining the brain in sections obtained by freezing, and I therefore purpose describing the method adopted by myself, feeling convinced that it will be fully appreciated by those who have up to the present employed the tedious process of chrome hardening, and suffered, as I have personally, from the numerous disappointments attendant upon the exclusive use of this method. It appears to have been the opinion of several microscopists who have employed the chrome salts exclusively, that the freezing method was wholly inapplicable to the investigation of the central nervous system, and several authorities in cerebral histology had expressed to me such an opinion. Results obtained by at least one German observer and my own work at the West Riding Asylum prove this view to be incorrect.

I was led to the employment of ether for freezing tissues, after having experienced numerous difficulties in the use of the ice and * 'Brain,' vol. i.

d

C

salt mixture. The process was so tedious that a more expeditious method appeared desirable, and I therefore constructed an instrument for freezing with ether spray, and described it in the 'Journal of Anatomy and Physiology' for April, 1877. At this time I was informed that an instrument was described by Mr. Hughes in the same journal twelve months previously; but finding my microtome simpler in construction and more expeditious in freezing, I have employed it, with some slight modifications, up to the present time. The woodcut represents in vertical section the freezing microtome which I constantly employ, and which I can very strongly recommend for general use. The lower half (a) is in principle an ordinary Stirling microtome; the upper half consists of a freezing chamber (b) and the section plate (c). As regards the microtome, I always insist upon the use of an oval instead of a circular plug (g), whilst the screw should be three-quarters of an inch in diameter, finely worked, and with a milled head at least of an inch wide. The freezing chamber (b) should have a false sloping bottom (f) leading to

h

3

a

an exit tube (e), which conducts off the condensed ether. Extremely simple as this contrivance appears, it is absolutely necessary for success that attention be paid to the following details of construction. The freezing chamber consists of a large hollow cylinder of zinc, slightly over two inches in diameter, and capped with a plate of the same metal. This cylindrical chamber is soldered on to the microtome plug, so as to ensure absolute steadiness in working, and it possesses three circular openings three-quarters of an inch in diameter, one placed in front of the two others laterally opposite each other. The section plate (c) is also made of zinc of an inch thick, and raised upon the vertical arm (d), also made of the same metal. The opening in the section plate should be sufficiently large to allow of the free play of the freezing chamber (b) through it without affording any point of contact between the two. With a freezing chamber such as the one described, beautifully large and thin sections may be obtained with ease. The material employed in the construction of the freezing chamber and section plate is a matter of importance, and I met with frequent failures and disappointments when endeavouring to utilize other more workable metals; brass above all metals was found unsuitable, and zinc alone fulfilled all the requisite conditions. Theoretically, it was supposed that the metal chamber should be covered with a non-conducting material, such as felt, wood, &c., and that the conduction between the section plate and body of the microtome should in like manner be cut off; but practically it was found that at the sacrifice of a small amount of ether rapid

freezing could be ensured and a large number of sections obtained before the tissues became loosened from the freezing chamber. It should be remembered, that to avoid the expense and incumbrance of a special condensing apparatus we have to provide for the free evaporation and subsequent condensation of ether in the same chamber, and consequently a sacrifice of about one-fourth the bulk of ether used is sustained. I regard as the requisite of a good freezing microtome the following conditions:

1. The instrument should be of the greatest possible simplicity. 2. The freezing should be rapid and expeditious.

3. The metallic constituents should be such as to retard thawing of the tissue when once frozen.

4. A minimum of ether should be expended.

Now, I would claim for my instrument a fulfilment of these conditions as far as is possible, without the employment of an exhausting and condensing apparatus; and I would on these grounds advocate its use by those who require a most satisfactory microtome, and one comparatively inexpensive. The first condition, that of simplicity, is too self-evident to dwell upon; the second also is ensured, as the tissue is frozen in less than twenty seconds, whilst it remains adherent to the cover of the freezing chamber for a period sufficing for the cutting of a dozen sections or more. A very small quantity of ether suffices for freezing, and three-fourths of its bulk becomes condensed, and may be collected in a bottle attached to the tube (e). I used a graduated bottle for the ether spray, and can thus read off the amount of ether expended in freezing. The costliness of ether has been urged against its employment in ordinary section cutting by one authority of note, and the objection would prove of great weight in case a large amount of ether was lost at each operation; it is but necessary, however, that the instrument be once seen in good working order to dispel all such notions from the mind, as the results obtained are of the first order, and the expenditure of ether very trivial."

The paper includes a description of the method of cutting sections of brain, but space prevents any further extract here, and the original may be profitably referred to.

Angular Aperture defined.-Professor Romyn Hitchcock, of New York, brought this subject before the Indianapolis Congress. order that the term "angular aperture" should mean something definite, and to avoid ambiguity and misunderstanding in future discussion on the subject, he proposed to adopt a definition of the term which, right or wrong, should be recommended to the microscopists of the country as a convenient and uniform usage. The triangle method was proposed for general adoption, considering the angular aperture of a microscope objective to be the angle of the apex of a triangle having a base equal to the available diameter of the front lens, and a height equal to the actual focal length (working distance) measured in air for a dry lens, and in the fluid employed for an immersion lens, the collar being adjusted for the most perfect definition in every case.

While nearly all the members seemed to be personally in favour