GENERAL CHARACTERS OF DIATOM ACEJE. 275 



circulation of the cell-contents of the Desmidiaceze (§ 201), has 

 been noticed by Prof. W. Smith* in some of the larger species of 

 Diatomacea?, such as Surirella biseriata, Nitzschia scalaris, and 

 Campylodiscus spiralis, and by Prof. Mas Schnitzel in Coscino- 

 discus, Denticella, and Rhizosolenia ; and although this movement 

 has not the regularity so remarkable in the preceding group, yet 

 its existence is important as confirming the conclusion that each 

 Diatom is a single Cell (the endochrome moving freely from one 

 part of its cavity to another), and that it does not contain in its 

 interior the aggregation of separate organs which have been ima- 

 gined to exist in it. 



213. The Diatomacea? seem to have received their name from 

 the readiness with which those forms that grow in coherent masses 

 (which were those with which Naturalists first became acquainted) 

 may be cut or h'olcen-througli ; hence they have been also desig- 

 nated by the vernacular term 'brittle-worts.' Of this we have an 

 example in the common Diatoma (Fig. 127), whose component 

 Cells (which in this tribe are usually designated as frustides) are 

 sometimes found adherent side by side (as at b) so as to form fila- 

 ments, but are more commonly met-with in a state of partial 

 separation, remaining connected at their angles only (usually the 

 alternate angles of the contiguous frustules) so as to form a zig-zag 

 chain. A similar cohesion at the angles is seen in the allied genus 

 Grammatophora (Fig. 128), in Isthmia (Fig. 134), and in many 

 other Diatoms ; in Biddulphia (Fig. 121) there even seems to be 

 a special organ of attachment at these points. In some Diatoms, 

 however, the frustules produced by successive acts of binary sub- 

 division habitually remain coherent one to another ; and thus are 

 produced filaments or clusters of various shapes. Thus it is 

 obvious that when each frustule is a short cylinder, an aggrega- 

 tion of such cylinders, end to end, must form a rounded filament, 

 as in Meloseira (Figs. 131 and 132) ; and whatever may be the 

 form of the sides of the frustules, if they be parallel one to the 

 other, a straight filament will still be produced, as in Ach.nanth.es 

 (Fig. 138). But if, instead of being parallel, the sides be some- 

 what inclined towards each other, a curved band will be the result ; 

 this may not continue entire, but may so divide itself as to form 

 fan-shaped expansions, as those of Lichmophora flabellata (Fig. 

 126) ; or the cohesion may be sufficient to occasion the band to 

 wind itself (as it were) round a central axis, and thus, not merely 



* The account of the Diatomacece given in this manual is chiefly based 

 on the valuable "Synopsis of the British Diatoniaceae," by the late Prof. 

 W. Smith ; of which, and of its beautiful illustrations by Mr. Tuffen 

 West, the Author has been enabled to make free use by the liberality of 

 Messrs. Smith and Beck. He has, however, entirely redrawn the sketch 

 which he has given of the Systematic Arrangement of the group, in ac- 

 cordance with .the more recent classification of Mr. Kalfs tPritchard's 

 " Infusoria," 4th Edition). 



t "Quart. Joum. of Microsc. Science," Vol. vii. (1859', p. 13. 



T 2 



