102 



KNOWLEDGE. 



March, 1913. 



bluish-green stain. In these cases they appear under the 

 microscope as fine threads of some shade of green, indistinctly 

 divided into numerous short cells, and when sufficiently free 

 exhibit the gliding and oscillating movements which have 

 given rise to their name. But the family contains also some 

 more specialized members which are less often met with. In 

 October last, I came across a solitary example of one of these, 

 clinging to a piece of Myriophyllum in a pond on Hampstead 

 Heath. In it a number of separate threads (trichomes) are 

 enclosed in a sheath -like case of gelatinous matter; they are 

 somewhat twisted and form a rope-like object. Individually, 

 they do not differ in any respect from many species of 



Figure 94. 

 Microcoleus subtorulosiis Breb. 



oscillatoria, but movement must be very restricted from their 

 close apposition and confinement within the investment. It 

 evidently belongs to the genus Microcoleus Des., 1823 

 {Cthonoblastus, Kiitzing 1843). West says of Microcoleus 

 — " The filaments are simple, terrestrial or aquatic in habit, 

 and are furnished with a conspicuous hyaline sheath. This 

 sheath is more or less cylindrical, not in any way lamellose, 

 and its apex is generally diffluent. The trichomes are 

 numerous, closely aggregated within the central part of the 

 sheath, and often spirally interwoven." My specimen agrees 

 very well with his M. subtorulosiis Breb, the thickness of 

 the trichomes being, as he states, about 5/i, the breadth of the 

 entire organism, including sheath, from 75 to 80/*. He says 

 " Plants of this genus are very rare in Britain." Dr. Cooke, 

 " British Freshwater Algae," and " Introduction to Freshwater 

 Algae," gives three species not corresponding very satisfactorily 

 with West's ; this specimen seems to resemble his M. terrestris 

 most nearly. The chief difference in the descriptions is that 

 he appears to imply that Microcoleus is frequent instead of 

 rare ; he says of its habitat, " on moist naked ground " and 

 " on the naked ground by roadsides," and so on. I have only 

 met with the plant once before, and on neither occasion in any 

 quantity, but it may occur more often in other parts of the 

 country than near London. If any of the readers of 

 " Knowledge " have been fortunate in coming across it in 

 greater abundance, a note with some details would be welcome 

 to me, and the Editor would, no doubt, be good enough to 

 insert it in " Microscopy." Figure 94 represents almost the 

 whole of the specimen referred to, the remainder being some- 

 what broken and disorganised. , „ 



5 James Burton. 



QUEKETT MICROSCOPICAL CLUB, January 28th.— 

 A. A. C. Eliot Merlin, F.K.M.S., sent for exhibition a photo- 

 micrograph at X 320 of Coscinodiscus heliozoides showing 

 " pseudopodia," from a preparation by J. D. Siddall. 



'Some notes on the Discoid Diatoms" were contributed by 

 W. M. Bale, F.R.M.S., of Victoria, Australia. The paper is a 

 survey and criticism of the principal characters which have been 

 utilised in the discrimination of species in some of the best 

 known genera of the discoid diatoms. Coscinodiscus, Actino- 

 cyclus, Asteromphalus, and Actinoptychus are dealt with. 



A paper on " British Freshwater Planarians (Rhabdoco- 

 elida) " by H. Whitehead, B.Sc, was read. While the British 

 marine Turbellaria have been monographed by Professor 

 Gamble, the freshwater forms of this country have received 

 but little attention. The classic monograph on the group 

 Rhabdocoelida is by von Graff. These organisms vary in length 



from about one to twelve millimetres. They are generally 

 found in ponds, lakes and ditches. The body is more or 

 less transparent, is slightly flattened and provided with cilia. 

 There is a mouth, a pharynx, and a sac-like gut. The position 

 of the mouth varies and affords a valuable generic character. 

 The excretory system consists of renal organs which in 

 some cases are somewhat complicated in character. The 

 nervous system is simple and comprises a two-lobed 

 brain and a pair of nerves running along the body close 

 to the ventral surface. Pigmented eyes are sometimes 

 present, and a statocyst (organ of equilibration) is occasionally 

 found. Reproduction is usually sexual. The animals are 

 hermaphrodite, but the male organs ripen first. Freshwater 

 Turbellaria undergo no metamorphosis. Reference was 

 made to the green chlorophyll-containing cells found in some 

 species. Professor Keeble has proved that in an allied 

 marine form, Convoluta, there is mutual benefit from the 

 association of these green cells (zoochlorellae) with the 

 animal. The Rhabdocoels are very difficult to prepare 

 in a satisfactory manner as permanent objects. The 

 most successful method is as follows : — The specimen 

 is placed in a watchglass with a little water, the bulk of 

 which is withdrawn by a pipette. A drop of Lang's fluid 

 is then delivered from a pipette on the side of the watch- 

 glass and allowed to run over the animal. After remaining 

 in the fluid for ten to fifteen minutes, the specimens are 

 removed to forty-five per cent, spirit, and are afterwards 

 passed through alcohol of increasing strengths, stained with 

 borax-carmine, and mounted in Canada balsam in the usual way. 

 In a note on Pleurosigma angulatum, E. M. Nelson, 

 F.R.M.S., stated that in spite of previously expressed opinions, 

 the apertures in the lower membrane of this and allied forms 

 can be unmistakeably seen below the intercostals of the upper. 

 Objective used was a Leitz one-twelfth apo. of 1-4 N. A. 



C. F. Rousselet, F.R.M.S., read a note on "Some Rotifers 

 from Devil's Lake, North Dakota, U.S.A." 



A DARK GROUND ILLUMINATOR.— Mr. E. M. 

 Nelson, F.R.M.S., has recomputed a dark ground illuminator 

 (see Figure 95) which is particularly useful for the examina- 

 tion of living bacteria. It is claimed that it produces a 

 brilliantly illuminated object on a velvet 

 black background. It can be used with 

 both dry and immersion objectives, but 

 the numerical aperture of the latter has 

 to be reduced by a small stop placed just 

 above the back lens. A funnel stop is 

 recommended, which can be fitted to any 

 immersion objective at a trifling cost, and 

 can be taken out quite easily when not 

 required. The great point of the 

 illuminator is that whereas those at 

 present in use require a powerful arc- 

 lamp, Mr. Nelson's can be used with an 

 ordinary oil lamp and a bull's eye con- 

 denser, provided that the work is done in a darkened room. 

 Mr. C. Baker, who is producing the illuminator, recommends a 

 Nernst lamp with a Nelson Aplanatic bull's eye condenser 

 attached, but incandescent and petrol gas lamps can also 

 be used. 



ON THE RELATION OF APERTURE TO POWER.— 

 A theory founded upon mathematical fact there is no gain- 

 saying, yet, while admitting the theory, one need not always 

 accept the proposed application of the principle. All praise 

 is due to Mr. E. Ardron Hutton for the care with which he has 

 prepared his article, under the above title ; for his clearness 

 in marshalling his figures, yet when it comes to marching 

 under his banner I find his colours are not mine. 



With his strictures upon opticians, each, according to him, 

 striving, in large apertures of the micro-objective, to offer 

 better bargains than his fellows, I am not concerned. True, 

 my own reading of their catalogues does not confirm these, 

 but the makers are quite capable of defending themselves, 

 and need no help from my pen. Then, however, he goes on 

 to say: "In the above case A (from offering the largest 



Figure 95. 



