488 .ANIMAL NATURE OF DIATOME^. 



have no positive data as to the internal organisation of 

 the genus. 



71. Mesocena. — Individua libera solitaria, annulum 

 circularem aut angulosmn sespe spinescentem referentia. 



On the five species of this genus we can only repeat 

 what has been said of Actiniscus. 



73. Dictyocha. — Individua reticulata spinosa, libera, 

 solitaria. 



Independently of the distinctive characters of the ten 

 species (one, the D. gracilis, being added by Kiitzing) 

 which merit attention by the variety of form, size, pro- 

 portion, disposition, and even number {D. aculeata, B. tri- 

 fenestratd) of the iyiiaglie) cavities, as well as the presence 

 and length of the spines, already noticed by Ehrenberg, 

 this author's observations on some of these species that 

 may be studied in a living state {aculeata, Speculum, Fibula) 

 are very valuable. From these it appears that the animal 

 is soft, and quite external to the siliceous body which it 

 bears on the surface, like a dorsal shield {scudo), as the 

 Arcellce bear their calcareous shell. 



Though observations are wanting to show that the 

 organisation proved only in some species of this last 

 genus (Bictyoclid), is common to the whole family of 

 Actiniscese, yet we may infer by analogy that it is so. 

 In fact, we cannot interpret the structure of those beings 

 in any other way than by supposing the body of the 

 animal to be external. 



And how slight soever, may be our knowledge of the 

 Dictyocha, it is still more than sufficient to prove an 

 organic type not merely quite different from that of the 

 other families comprised in the same order (Appendiculatse), 

 and in the same tribe (Areolatse), but even from the 

 type of all the other Diatomese. 



I believe that whatever be the rank assigned to the 

 group of Diatomese in the Zoological series, the first 



