DIATOMACE^. 



[ 199 ] 



D1AT0MACE.E. 



D. elongatum. Frustules very slender, 

 slightl}' attenuated towards the middle ; 

 valves linear, evidently striated, tumid and 

 rounded at the ends; length 1-280". Aquatic. 



D. pectinale. Front view square or ob- 

 long ; valves acutely lanceolate ; unstriated 

 (under orchnary illumination); length 1-/20". 

 Aquatic. 



Six other species (Kiitzing). 



BiBL,. Ralfs, Ann. Nat. Hist. 1843. xi. 

 p. 449 ; Kiitzing, Bacill. p. 47 ; Spec. Alg. 

 p. 16. 



DIATOMACE^.— AfamilyofConfervoid 

 Algae, of very peculiar character, consisting of 

 microscopic, brittle organisms, found in almost 

 all fresh, brackish, or salt water; sometimes 

 forming a uniform yellowish-brown layer on 

 the bottom of the water, at others adhering 

 to various water-plants, decaying stems, 

 stones, &c., or scattered between the fila- 

 ments of Confervae, &c. They also occur 

 among Mosses, Oscillatorice, and on damp 

 ground. 



The individual cells of the Diatomacese 

 are called frustules or testules, and are fur- 

 nished with an external coat of silica. This 

 consists of two usually symmetrical portions 

 or valves, comparable to those of a bivalve 

 shell, but are in contact at their mai'gins 

 with an intermediate piece (the hoop), vari- 

 able in breadth according to age. When this 

 is very narrow, it forms a mere junction line, 

 and is called the line of suture ; and that 

 aspect of the frustules in which this is turned 

 towards the observer forms the front or front 

 view (primarv side, Kiitzing, secondary side, 

 Rabenh.) (PI. 11. fig. /• ; PI. 12. figs. 9 «, 

 30 b). That aspect of the frustules in which 

 the surface of the valves is turned towards 

 the observer, forms the side or side view^ of 

 the frustule (secondary side, Kiitz., primary 

 side, Rabenh.) (PL 11. fig. 6; P1.12.fig.30«). 



The separate valves are of various forms, 

 circular, oblong, elliptical, linear, saddle- 

 shaped, boat-shaped (navicular), undulate, 

 sigmoid, &c. (PI. 11, 12, 13); and their broad 

 surfaces exhibit various more or less delicate 

 sculpturings and markings, in the form of 

 bands, lines either parallel, radiate, or cross- 

 ing each other, and dots, or a cellular ap- 

 pearance. 



These markings are in general not well 

 seen, and in some cases cannot be seen at all, 

 until the valves have been properly prepared. 

 They are of special interest, not only on ac- 

 count of their extremely beautiful symmetry, 

 but because they are used as test-objects for 

 the quality of the object-glasses in regard to 



angular aperture. The nature of the mark- 

 ings is described under the imhvidual genera. 

 The modes of viewing them will be spoken 

 of further on. 



During the process of multiplication by 

 division, which is almost always going on, 

 the annular, siliceous, narrower or broader 

 band, or hoop, undergoes an increase of 

 width, and thus removes the two valves to 

 some distance apart (PI. 11. figs, la, 11 ; 

 PI. 12. fig. 1). The hoop is sometimes fur- 

 nished with the markings, at others not. 

 Some of the valves are furnished with pro- 

 cesses, called cornua or tubuh (PI. 12. fig. 

 30 6); at others their surfaces are undulate, 

 producing the appearance of dark, curved or 

 wavy lines (PI. 12. figs. 22, 23, 24) ; some- 

 times curiously arranged lines (vittae) indicate 

 imperfect internal septa (PI. 12. fig. 35). 



In the young state of these organisms, the 

 endochrome is uniformly distributed, but 

 after a certain time the colouring matter be- 

 comes accumulated into various, usually very 

 regular and often elegant forms, and minute 

 granular globules are formed, transparent ve- 

 sicles become visible, drops of oil, and vesicles 

 filled with granules, which at first are mo- 

 tionless, but afterwards move about as in the 

 swarming motion of the Algae. Frequently 

 a considerable nuclear-looking body is pre- 

 sent in the middle of the frustule (PL 11. 

 fig. 33 a). As we have seen it, delicate pro- 

 cesses were visible arising from it. 



The frustules of the Diatomaceae are some- 

 times smTOunded by a transparent gelatinous 

 sheath, frequently of great delicacy ; in some 

 genera they are attached by a stipes or stalk 

 to water-])lants, &c. 



Those Diatomaceae which are not fixed by 

 a stipes, and especially such as are linear or 

 spindle-shaped, are capable of spontaneous 

 motion ; they may be constantly seen slowly 

 moving across the field, or now and then 

 starting somewhat suddenly forwards, moving 

 mostly in the du'cction of their length, some- 

 times receding, sometimes performing a ro- 

 tatory movement on their axis. Those which 

 are contained in numbers in a gelatinous fila- 

 ment, like Encyonema, are capable of moving 

 backwards and forwards in this; and Mr. 

 Thwaites described a curious movement of 

 the frustules of Bacillaria paradoxa, where 

 the frustules, united in a band, slid back- 

 wards and forwards over one another. 



The cause of these motions is very ob- 

 scure. They have been supposed to be pro- 

 duced by the endosmotic changes connected 

 with the nutrition of the organisms, but this 



