4 6 



The parenchyma (PI. VII, figs. 3 and 4) is of the same sort as in 1945 viz. lymphoid, 

 forming a reticulum of very fine fibres and flat cells with extremely thin membranous expansions. 

 In this ground-tissue cells of various aspects occur, now more, then less. Seen with low power 

 the membranous expansions simulate threads ; seen with higher power and after carefully focussing 

 it becomes evident, that the branching thread-like prolongations are indeed the optical sections 

 of membranes l ). This tissue is especially well visible around the larger canals in the central 

 parts and of course lining the central canal. Whereas in other parts it is confused by numerous 

 cells of various description — amoebocytes, thesocytes etc. — or by spicules, both these elements 

 are scarce or absent in the neighbourhood of large canals and the central canal. Among those 

 cells we find elongated fusiform cells, probably contractile ; but also we find thread-like cells 

 with very much elongated nuclei (PI. VII, fig. 2), having absolutely the character of true muscle- 

 cells ; they may reach a length of 1 / 3 mm. Now these cells are abundant in the tissue lining 

 the central canal ; here they are found in bundies, concentrically arranged, but also radially 

 (PI. VII, fig. 4). They are easily recognised by their characteristic, rod-like nuclei. All the 

 canals seem to be covered by pinacocytes. The external surface I believe to be protected by 

 a kind of cuticula, the mother-cells of which I could not distinguish with certainty. 



The skeleton is formed by longitudinal bundies of tylostyles, elliptic or flattened on 

 transverse section. In the wall of the tube we find three or four bundies: one close under the 

 lining tissue of the central canal, one at some distance from the external surface and one 

 or two in the middle. The bundies may give off branches with anastomose with other main 

 longitudinal bundies. At more or less regular intervals the latter send off bundies in radial 

 direction ; these give rise finally to the diverging superficial tufts or brushes, the spicules of 

 which are generally shorter than those of the main skeleton. In some places tylostyles are seen 

 irregularly dispersed in the meshes of the main reticulum. Spinispirae are absent. In 1945 we 

 saw the ectosome strengthened by a crust of spinispirae and some tylostyles; in 1365 only the 

 latter are present, but they are far more numerous, slightly projecting and hence making the 

 surface more or less hispid. I did not find any tracé of spongine. Whether the groups of large, 

 ellipsoid, sometimes fusiform thick cells with coarse granules, accompanying the main bundies 

 of tylostyles have anything to do with the formation of spicules or other elements of the 

 bundies, I could not make out. It is, however, probable. 



3. Specimens of different tropi compared. 



If we make a longitudinal section through a digitate process of a typical example of 

 tropus digitata, e. g. 948 g, we see a large central canal, surrounded by a crumb of bread like 

 substance, which is perforated by canals of various diameter, running in different directions 

 (PI. V, fig. 1). The central canal is generally filled with a muddy substance, containing débris 

 of little shells, Foraminifera etc. We find these likewise in the larger peripheral canal as is 

 shown in Fig. 2, which represents a transverse section of 948 b. If these objects are removed 

 and thus the inner wall of the canal cleaned, the latter is seen to possess concentric folds or 



1) See about this tissue Vosmaer, 1908 (a) p. 41. 



