THE MONTHLY MICROSCOPICAL JOURNAL. SEPTEMBER 1, 1870. I.-On the Structure of the Pleurosigma angulatum and Pleurosigma quadratum. By JOHN ANTHONY, M.D., Cantab. (Read before the ROYAL MICROSCOPICAL SOCIETY, June 8, 1870.) PLATE LIX. I HAVE to call attention to a peculiarity in the structure of the P. angulatum and its allied species, which, so far as I know, has never been described. Taking the fact that few microscopic objects have been so frequently and so critically examined as the silicious valves of these beautiful diatoms, not only for the purpose of determining the precise nature of the markings which characterize the Lorica themselves, but by way of testing the appliances of the microscope, it might fairly be expected that we had fully made out and put on record all that was worthy of note with respect to these shells, but that such is by no means the case I trust to be able to show clearly in the course of this paper. I will begin by saying, that I feel grateful to these minute objects for the lessons they have taught me in the use of the microscope; more particularly in the management of light so as to distinguish between the true and the false; and that I have studied these silicious scales for years, from the far-away time when I was proud of being able to make out crossed lines on their surface with the aid of one of the earliest ths constructed, until now, when I am even more pleased to be able to show the nodules, or "dots," as we used to call them, most brilliantly by means of a modern th. Having then studied these objects so long and so constantly, I trust it will not be thought that the observations I am about to make are either hasty or given without due consideration. The Pleurosigmata have usually been divided into two sections, viz. into those in which the striæ or rows of dots are arranged "obliquely" with respect to the axis of the shell, and into those which are arranged "transversely and longitudinally," or as I should prefer to call it "rectangularly." In the first or oblique division, the Pleurosigma angulatum and P. quadratum have been unhesitatingly placed, while the second or rectangular division has received P. hippocampus, P. Balticum, and others well known to VOL. IV. K the microscopist. Now if I am right in my observation, this classification of the markings of P. angulatum as oblique-that is wholly oblique is erroneous, as I find the markings, strangely enough, to be of a mixed nature, and it is to this peculiarity I wish to call attention. I have made out most satisfactorily, that in the said P. angulatum and P. quadratum the ends of the valve, for about one-sixteenth of its length, have not any markings but such as are distinctly rectangular, markings of the character in fact of those seen in the second division, as P. hippocampus, P. Balticum, &c., the striæ being, as in these last, farther apart transversely than they are in the direction of the axis of the shell. It must not be assumed that these markings are either accidental or few in number, for not only have I found them to be always present, but in what perhaps I may be permitted to call a normal or symmetrical valve, I have counted 13 rows of dots from the mid-rib towards each edge, while I made out the number as 32 to 50 dots arranged in these lines between the obliquely-marked portions of the valve and its end. In the sketches which accompany this paper, I have drawn with the camera lucida an entire Lorica, and indicated exactly the bounds respectively of the oblique and rectangular arrangements. The larger drawing (Fig. 1) represents the appearance of the end of one of these chosen Pleurosigmata under the most favourable conditions of light and definition, and when, as is by no means always the case, the valve is symmetrical. This symmetry is so little constant, that I should venture to say, that the silicious envelope seems subject to what gardeners usually call "sports," that is, that in all the ends having the rectangular markings I have attempted to describe, scarce any two of the specimens would be exactly alike. For instance, the portion of valve on one side of the mid-rib would be rectangularly marked to double the extent of the other, in which case I generally found that the other end of the shell repeated the peculiarity. On a few occasions, I have found the curious "sport (Fig. 2) which I have indicated in one of the sketches, where the longitudinal close series of dots forming striæ, bend at right angles without a break, and so become transverse; the next series follow the example, till the whole set has somewhat the appearance of a set of carpenter's squares. I think I am not deceived in my observation, that these rectangularly arranged "dots" are really larger in size, or as we are accustomed to call it "coarser," than the usual and well-known "dots" which are arranged obliquely, and they are not quite so easy to resolve, and to deprive of spectra or shadows. Now I do not want anybody to take this peculiarity of structure in our old friend P. angulatum for granted; I trust that each microscopist will examine for himself, it being only fair to say, however, that to make out this structure well, will require the very best appliances, and no small amount of care in the use of them. I feel sure that the structure is there, that it is not an optical deception, and that a fine instrument, a careful manipulation, and a good sight will not fail to make it out. I may say here that I prefer for such an examination a 4th or a 1th to ath or ath, for the reasons we are now beginning to understand, viz. that we can, with the lower objectives, "take cognizance of more planes ;" and it must not be forgotten that the ends of the P. angulatum and quadratum are curved or twisted. Also I may observe that, working as I always do by preference with a rectangular prism instead of a mirror, and consequently with the light directly in front of the microscope, I find that the markings I have indicated are considerably more distinct when the valve under examination is placed vertically or nearly so. I trust the Society will not consider these details trivial, for independently of making out more perfectly the structure of an object which has always been deservedly a "pet" with microscopists, we are beginning to recognize that we are in a transition state with regard to our knowledge of the true structure of hundreds of objects which were formerly misunderstood, mainly on account of the material in which it was thought necessary to mount them, greater attention having been paid of late to quality rather than to quantity of light for illumination, and the blaze, and the glare, and the "pretty" but false appearances produced by Canada-balsamed objects having been relegated to the class of observers who look on the microscope as a sort of peep-show, instead of a most valuable instrument for scientific research, and towards the improvement of which I am glad to feel so many minds are now bent, each humbly seeking for truth, and none daring to claim even an approach to perfection. EXPLANATION OF PLATE LIX. FIG. 1. Extremity of P. angulatum (reduced from original drawing). 2. Variation of rectangular marking on P. angulatum (reduced from original drawing). 3. P. quadratum, drawn with camera lucida, to show relative extent of oblique and rectangular markings (reduced ). |