12 6 



E. Jørge 



Haeckel considers those forms which have a marked principal 

 axis to be „monostatic", supposes that they swim in one definite 

 position of balance. As the skeletons of Nassellaria with their 

 substantial spines and net walls must be considered decidedly 

 heavier than water, their plasmatic parts are most probably lighter, 

 in order that the form may be able to float in smooth water, when 

 no selfmotion is supposed. 



It is another matter, whether it follows from this that tliese 

 forms are monostatic, as Haeckel supposes. It seems, however, 

 natural to conclude that the plasmatic parts support the skeleton 

 and not the reverse, so that it would appear most likely that the 

 central capsule is situated under, not over „the tripodium" (in those 

 forms which have an imperfect skeleton). 



If, however, the centre of gravity of the plasmatic parts is 

 under „the tripod", it is not impossible that the centre of gravity 

 of the tripodium, by which, naturally, the basal position is deter- 

 mined, may coincide with that of the plasmatic parts, and thus 

 the balance in the water be indifferent. 



One would think that such a position of balance would provide 

 important advantages and would safeguard against a separation of 

 the central capsule from the plasmatic parts of the skeleton. 

 There are, moreover, several details concerning the different Nasael- 

 hirin skeletons which seem to suggest that the centre of gravity 

 of the skeleton is very near that of the plasmatic parts. Thus the 

 substantial „tophorn" (Hck.) which is found in numerous forms of 

 Cjjrtoidea may be mentioned as an instance of this. It is especi- 

 ally this solid tophorn which makes it appear not improbable that 

 the centre of gravity of the skeleton, if it does not coincide with 

 that of the plasmatic parts, even might well be nearer the cephalis, 

 so that the form when thoroughly balanced ^\■o\M swim witli the 

 cejihulis and the tophorn downwards. 



In this connection, it is an interesting fact that the central 

 capsule in several Cyrtoidea — e. g. Clathrocijdas, Corocalyptra, 

 Dicfi/ophlinits (cfr. Haeckel, L. 86, pi. 60, 61) is lobed, and has 

 a small part in the cephalis, while four long, widened bags extend 

 thi'ough a large part of the thorax. If these bags are lighter than 

 water, their natural position in the heavier skeleton will be up- 

 wards (although the species, of course, would also swim steadily 

 with the cephalis upwards, if only the centre of gravity of the 

 skeleton lies under that of the plasmatic parts). 



Here I will not omit to add that this peculiar condition of 

 tile central capsule in Gyrtoidea suggests an original placement in 

 till' cephalis, while later on it has got its greatest volume in the 

 thniiix. Tills again points decidedly to the probability that those 

 forms wliicli have an imperfect skeleton are to be considered as 

 Munocyrtida with an imperfectly developed cephalis (and no thorax). 

 The apical arches and their secondary arches in Phormacantha 

 form a continuation of the net work up over the sides of the 

 a I ileal spine A, thus causing an enclosed dorsal spine, i. e. a ,,col>i- 

 iiirlla" (HcK.), as in Euscenium Hck. and Cladosceniuni Hck. The 

 (vlionella is as yet only partially embodied in the skeleton. At 

 tlie same time a further development of the opposite part takes 

 place, a rather complete wall being formed outside the ventral arch, 

 below the ventral sagittal spine, if this is considered as directed 

 downwards. Probably this is to prevent that the centre of gra\ity 

 of the skeleton shall be moved to one side. 



The genus Cladoscenium, as it is represented by C', trirolpium, 

 with an almost central columella and even development of the 

 skeleton all around it, may quite plainly be derived from an 



figure) and finally .3 arches bet- t 

 ween the nearest branches of 



1, 

 k.L\Å 



Fig. vin. 



obli( 



interesting, simple form, Protosceninm, which can again be traced 

 back to the Plagiacantha type. We may imagine the original type 

 with the four main spines developed evenly, and having the 

 primary verticil of branches on each of them placed in regular order. 

 (Cfr. fig. VIII). If one now imagines 4 basal arches formed between 

 the nearest branches of the dor- 

 sal spine, D, on the one side, 

 and each of the lateral spines, 

 L, on the other (cfr. the lines 



.... on the figure), 2 similar ./ ^^ / ^. 



arches between the nearest / Å n \. 



branches of the apical spine. / / \ ■ 



A, on the one side, and the / ...--■■.•' ■■■••■•... \ 



dorsal spine on the other (cfr. 

 the lines — — — - on the 



■ ^'^ L 



_ .jj. . _-.e Protosren'ui,m-{\\w. 



the apical spine and the lateral oblique apical view 



ones, (cfr. the lines — — — 



on the figure) one has the Protoscenium type. Here there is an 



almost central columella, which naturally presents itself as an „ apical 



spine" in contradistinction to the other three, which are all about 



equally developed and want the outer branch in the primary verticil, 



while in the columella all three branches are well developed. 



The columella, however, also here corresponds to the dorsal 

 spine in the foregoing species. This will also be seen if we regard 

 the natural position of the central capsule, between D, L and Li 



From Protoscenium simplex to Cladoscenium tricolpium, we 

 have again a very plain transition. The branches of the primary 

 spines lose their tips and become parts of the arches (as in Phorm- 

 acantha). The three branches from the spine D are the verticil 

 on the apical spine (after Haeckel). A ventral sagittal spine is 

 also developed, and in this way, perhaps, a counterbalance is formed 

 to the net work at the spine A. 



From the Plectacantha type a series of forms may again be 

 traced. The net work closes at the base (cfr. corresponding process 

 in Ceratospyris) and we have the genus Peridium, which always 

 seems too to have the ventral, sagittal spine whicli often is, how- 

 ever, but weakly developed. One can here, partly because of the 

 meshes round the dorsal spine, more especially because of the 

 characteristic peculiarity in the position of the ventral and left 

 lateral arch, (suspended from a common, basal, primary branch 

 from the left lateral spine, cfr. Pkctacanthai be convinced tliat 

 Haeckel's ,,apical spine" corresponds to tlie Imsal dorsal spine, D, 

 in the genera with imperfect skeleton. ( )ir' linds also in Peridium 

 several of the characteristic peculiarities of Plertacantha oikiskos, 

 e. g. the larger, secondary meshes with their conspicuous byspines. 



If the connection with Plectacantha oikiskos may be taken as 

 certain — there is, perhaps, an intermediate form, the one already 

 mentioned which resembles Plectacantha oikiskos, but has a ventral 

 sagittal spine — the connection between Peridium and Cyrtoidea 

 is at any rate quite as certain. Here, however, as mentioned above, 

 the ,,cephalis'' corresponds to the basal network of the simpler 

 forms. One can plainly see in young Cyrtoidea, where it is prin- 

 cipally the cephalis, which is fully formed, how the thorax is devel- 

 oped betAveen the outward pointing- liysjiiucs on the ininiaiy and 

 secondary arches of the cephalis. 

 j In Lithomelissa setosu, whicii can quite naturally be derived 



1 from Peridium, the spine A luns inside the thorax, wliich it then 



