DIATOMACE^:. 



419 



Diatomacece, brittleworts, siliceous acilla?'ia, are organ- 

 isms composed of two symmetrical plates or valves, narrow 

 or wand-like, navicular as a miniature boat or "little 

 ship ; " hence their name, Navicula. A rectangular or 

 prismatic figure is, however, the typical form of this 

 family, and the angles of junction of the valves are as a 

 rule, acute. Deeply notched frustules, such as we see in 

 the Desmids, Micrasterias denticulata, Docidium pristidce, 

 Plate II. Xos. 30 and 31, do not occur, and the produc- 

 tion of spines and tubercles is rare among the Diatoms. 

 Each individual Diatom is enclosed by a soft organic 

 matter (sarcode) ; the internal portion is yellowish or 

 orange-brown in colour. In the discoid forms two por- 

 tions are commonly distinguishable, viz. the disc and 

 margin or rim, and these present different markings, with 

 an occasional central prominence, called an umbo or boss. 

 Great variety of outline may prevail in a genus, so much 

 so, that no accurate definition can be safely 

 laid down : thus in the genera Navicula, 

 Pinnularia, the frustules are in one aspect 

 boat-shaped, and in another oblong with 

 truncated ends, prismatic. Mr. Brightwell 

 thus describes and explains the transitions 

 of form produced by a change in position 

 of the frustules of the genus Triceratium. 1 

 " The normal view of the frustule may be 

 represented by a vertical section of a tri- 

 angular prism. If the frustule be placed 

 upon one of its flat sides, we look down 

 upon its ridge and obtain a front view of 

 its two other sloping sides. If it be placed 

 upon one of its ridges, we have a front' 

 view of one of its flat sides, generally 

 broader than long, and of its smooth or 



,. Fig. 221. Gompho- 



trausparent suture or connecting mem- nema eiongatum 

 brane. If the frustule be progressing ** wpitatum. 

 towards self-division, it is then often considerably longer 

 than broad, and when nearly matured for separation, pre- 

 sents the appearance of a double frustule." So with re- 



1) Journ. Micros. Soc. vol. I. p. 248. 

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