68 



As illustrations we have chosen here the following representatives 

 of these two classes: Of the group Z>f : Biddulphia pulchella (fig. /); 

 Auliscus elegans (fig. 2); Navicula dichyma (fig. j); of the group 

 Z)?: Triceratium digitate (fig. 4), and Robertsianum (fig.s)', Actino- 

 ptychus constellatus (fig. 6}. 



Of the groups Z>f and D*% we have chosen as examples the 

 crystal-forms of zircone: ZrSiO^ (fig. 77; D H 4 }, and of beryll: 



Be 3 Al z (Si0 3 ) 6 , (fig. 78; !>?.) 



On Table 77 moreover the following objects have been reproduced 

 of D^: Actinoptychus heliopelta (fig. 7); Amphithetras elegans (fig. 8); 

 Auliscus crucifer (fig. p) and cratifer (fig. 10). Of the group Cf 

 only Aulacodiscus Grevilleanus (fig. //); a very fine specimen of 

 this symmetry being also Triceratium pentacrinus, which is however 

 not reproduced here. The form of Grovea pedalis (fig. 12) on the 

 contrary may be looked upon as an illustration of the group D, 

 whose representatives are very rare. Most of these figures were 

 reproduced after Haeckel's original drawings. 



Of the group D*j no instances have been found among the Dia- 

 tomeae; but perhaps some radiolaries may belong to this class: so 

 Ethmosphaera siphonophora (Haeckel). // the top and the basal 

 parts of the fruit of Badamia Commersoni (fig. 7/) were equally 

 developed, which is certainly not the case however, this form 

 would give an idea of a vegetable object possessing this symmetry. 



Some pollen-cells, e.g. those of Passiflora augustifplia, Heliotropum 

 grandiflorum, etc., may be reckoned, according to some authors, to 

 have this symmetry too. 



The spicula of a number of radiolaries are arranged in a regular and 

 most remarkable way. This special regularity was already described 

 by Joh. MuTler, and defined by him in a "rule" which bears his name. 

 The particular symmetry of this arrangement appears to be that of 

 the group D*j ; as in the case of Acanthostaurus, Acanthometra, etc. 

 The pollen-cells of Annona tripetala seem to belong here too. 



A striking example of the symmetry D*j in the case of radiolaries, 

 is that of Acanthodesmia prismatium (fig. 79), after Haeckel's 

 drawing. 



Haeckel was one of the first to understand the eminent value 

 of the principle of symmetry for the description of plants and animals, 

 and to draw special attention to the symmetrical and highly aesthetic 

 forms of the lower beings in his great work on the Radiolaries, and 

 in his book: "Kunstformen der Natur", 



