124 



E. Jørgensen. 



any form in ^\'hieh it was wanting. Its comparatively weak de- 

 yelopment, in many cases, may tlierefore be due to retrogression. 

 vVt any rate, it would have to be looked upon as a spine which 

 appears at a very early stage of development, so that, in the case 

 in question, it will only be immaterial whether it is considered as 

 being formed at the same time as the 3 basal ones, or as having 



u 



7ig. II. The ground form ; 

 ai)ical view. 



Fig. in 



Tlie same; ventral 



D 



A 



been formed later. In other words, Haeckel's basal tripodium may 

 be a more original form; but the ground form with 4 spines which 

 I have chosen as a starting point, is the common type for all the 

 forms which I have found. 



This primary verticil of branches is found, more or less di- 

 stinct, in a series of forms, which appeal-, therefore, to originate, 

 mure or less directly, from the Plagiaccmiha type. 



There is a peculiarity with regard to the verticil, whicii plays 

 an important part when examining the skeletons, as it makes it 

 easier to trace the corresponding parts and makes clear the rela- 

 tion between certain forms. 



The branches of the dorsal spine, D, 

 are placed one dorsally, two ventral-later- 

 ally. (Here we only regard the projections 

 of the branches on a plane perpendicular 

 to the direction of the main spines. The 

 actual direction of the branches is deter- 

 niinod too liy tliat of the main spines, so 

 that it will be, respectively, dorsally up- 

 wards and ventral -laterally downwards.) 

 On the right lateral spine, the direc- 

 ^. ,,, „„ ,„ . „ , tioii is almost corresponding, that is with 



Fig. IV. The Plagiacantha tvpo; ^ •" 



transverse section of the basal One branch outwards (away from the cen- 

 •spines. tral 1-od, cfr. fig. IV), while the verticil 



of the left lateral spine is turned so as 

 to take the position as denoted on tig. 1\'. 'I'he edges liave also 

 tlie same position, where they occur, as well as the secondary 

 spines. 



In this Avay, tlie plasmatic body will be more perfectly jiro- 

 tected, than would be the case if all three verticils were in the 

 i-egular positions. 



A development in two directions now takes place. I'here 

 appears a oth, important, spine, the ventral sagittal one (in my 



A- 

 1,. 



X 



pre^^ous paper, .Ioegensex L. 91, designated as ,.der secundilre 

 Mittelstachel"), and the primary connecting arches appear. 



The ventral sagittal spine extends from the common starting 

 point for the two lateral spines (the ventral end of the central rod) 

 and is also basal (directed downwards and for'i\-ards). On those 

 skeletons which are less richly developed, it is easy to i-ecognize 

 it as a less strongly developed spine of later origin. 



The primary arches are developed between those primary 

 branches of the basal spines which are nearest each other, and 

 are, therefore, three in 

 number: (1) The left la- 

 teral arch. l!i tig. V, bet- 

 ween the left branch of 

 the dorsal spine. (1| and 



the inner branch, Ijj, of the 



left lateral main spine; 



(2) the ventral arch. 1^^. 



between 1^ 

 branch, 1^.] > 



lateral main 



(3) the right, 

 B„ between 



branch, 



h'd. 



ioht branch of the dorsal one. d„ 



- de- 

 being 



lateral main spine and the 



Thus, the ventral arch and the left, lateral arch run together 

 to a point on b-, and consequently have a corner in common which 



is suspended under the left, lateral main spine. 



If one no\\- supposes the formation of a more complete net- 

 work through tlie addition of secondary meshes and spines, in 

 basal direction, on the sides of the pyramids, one has, in all 

 important respects, Plectamntha oUaslvs. 



On the other hand there is — as above mentioned 

 velopment in another direction, the ventral sagittal spine, V 

 formed. 



iV simple form 

 which belongs here 

 is Cuvqijjluraufhn 

 cladophora. In its 

 most simple state 

 (young forms?) it 

 consists of the 5 s])i- 

 nes D, L,. L]_ A 

 and V (cfr. lig. VI 

 and lig. Vll). The 

 most fully developed 

 individuals seem be- 

 .sides to have Iwd 

 secondary latfi'al 

 spines, directed la- 

 teral-dorsally and 

 basally, 1,, and 1], ex- 

 tending fniiii the 

 ba.se of the apical 

 spine. (These spines, 

 1, are perhaps only 



