( 1145 ) 
this way a reticulum is formed, the meshes of which are very 
different in size. In addition to these cells, fibres oeeur beyond doubt; 
perhaps also a kind of elastic fibres. For the rest we find cells of 
various description: amoebocytes, thesocytes, fusiform cells ete. The 
canals are lined with flat cells, which have exactly the character of 
the first mentioned cells; they may be considered as endothelium. 
The skeleton is mainly formed by bundles of tylostyles. In 
encrusting specimens these bundles stand vertically on the substratum; 
they ramify generally towards the periphery and at any rate terminate 
in brushes. The spicules of the latter are usually smaller than the 
tylostyles of the main bundles. In massive specimens or those with 
finger-shaped processes long longitudinal bundles run through the 
parenchyma, here and there ramifving, occasionally anastomosing. 
These main bundles give off smaller branches towards the periphery; 
these as well as the main bundles terminate in more or less pro- 
jecting brushes. Hence the sponge surface is now rather hispid, then 
smooth. On the whole we can say that the number of superficial 
tylostyles is in reverse ratio to the number of spinispirae. If the 
latter are abundant they form a “dermal” crust. If in addition to 
minute spinispirae robust ones occur, this crust is composed of one 
or two distal layers of the former and 2—5 proximal layers of 
the latter. 
In many specimens with well developed longitudinal bundles, 
strings of darkly stained cells are seen at once in every preparation. 
These cells are more or less fusiform, possess a large nucleus and 
a large “nucleolus”. They are found in close connection with the 
bundles of tylostyles. Most probably these cells are fibroblasts; 
they are the cells which form the connective tissue fibres, which 
strengthen the bundles of tylostyles by binding together the spicules. 
This tissue I have called periapt'); herein occur, in addition to 
fibres, fusiform cells etc. These fibroblasts which thus give rise to the 
fibres, are not always situated in such conspicuous strings; but they 
may be found everywhere, where fibres are to be formed or are 
normally present. 
The occurrence of spinispirae, the arrangement of the canal system, 
the whole anatomical structure of Spirastredla, all suggests a close 
relationship to the so-called Boring Sponges, belonging to the genus 
Cliona. The two genera are, however, distinguished from each other by 
the fact that Chona  perforates calcareous matter (shells, corals, 
coralline algae ete), whereas Spirastrella does not bore. Several speci- 
1) Proceed. Kon. Akad. Wetensch. 1905, p. 25. 
