75Q SYSTEMATIC HISTOEY OF THE INFUSORIA. 



Sub-group DIATOMEJE or DIATOMACE.E. 



(Page 31, Plates IV. to XYII. and part of II.) 



[For reference to tlie species figured in this work, see Index of Diatornace£e 



illustrated.] 



Frustules or cells, either simple or pseudo -unicellular by complete separa- 

 tion, or united in tablets or filaments, fm-nished with a sculptured siliceous 

 coat in three portions, a median one (connecting zone) and two lateral ones 

 (valves) united by distinct sutures ; internal substance yellowish-brown 

 (rarely olive-brown) ; reproduction by conjugation and subsequent formation 

 of sporangia. 



The general histoiy of the Diatomacese has been so fully treated of in the 

 first part of this work (p. 31) that it is here only necessary to explain some 

 teiTQS used in the descriptions. 



The Diatomaceae differ, in several respects, so widely from acknowledged 

 Alga?, that in our opinion ihej maj^ be regarded rather as an order related to 

 the Algse than as a family belonging to them. 



The siliceous covering is composed of three portions. The central one is 

 sometimes called "connecting membrane" and "cingulum;" we, however, 

 prefer Professor Amott's term, " connecting zone," as less likely to mislead. 

 The lateral or junction surfaces correspond to the septa of a Confeiwa, and are 

 called valves. 



The late Professor Smith considered the central portion unessential and 

 produced onty preparatory to self-fission. "VVe, on the contrary, regard it as 

 of great importance, and quite unkno^Ti in the true Alga). It is conspicuous 

 in the conjugating and, consequently, matm-e frustules ; and we think the con- 

 clusion illogical that it has no sj'stematic value because obscure in newly- 

 formed frustules. It is evidently essential in Diatoms with flat valves, since 

 otherwise there could be no cavity to contain internal matter. 



We use the term " front view " to denote that position of the frustide 

 when the connecting zone is fully presented to the e^^e, and " side view " 

 when the centre of the valve is in a similar position. A\Tien we speak of the 

 " valve," unaccompanied by a qualifying epithet, it must be understood as 

 identical with " side view." 



''Longitudinal" means in the direction of the connecting zone, and 

 '' transverse " in the opposite direction uniting the valves. When so applied 

 to the frustule of a Diatom, these terms acquire a meaning exactly the reverse 

 of that in which they are used when applied to the joint of a Confei-va and 

 the frond in the Desmidie£e. For example, the frustnle in some Diatoms and 

 the frond in Closteriiun are both described as longitudinally lunate, whilst 

 they are really extended in opposite directions : unless the change in the 

 meaning of the terms be remembered, an idea of similarity will be conveyed 

 which is altogether erroneous. 



The valves are sculptured, cellulose, or striated ; the apparent absence of 

 striai in some instances may be accounted for by their extreme delicacy 

 placing them beyond the reach of our instruments, since the greater the 

 penetration of the object-glass, and the more perfect the illumination, the 

 greater is the number of species found to possess them. AMien, therefore, we 

 use the terms " smooth " and '' very smooth " in definitions taken from foreign 

 works, they must be understood to mean only that the stride were too fine to 

 be ascertained by the microscope of the describer. 



