402 Dr. O. Maas on the Interpretation 



reality tliey are only phases of contraction, which pass some- 

 what quickly into one another ; and if the ectoderm be 

 investigated in different stages of contraction all gradations 

 are found, from an ordinary flat cell (where the sponge is 

 expanded) to completely mushroom-shaped cells, which show 

 the chief mass of the cell- body displaced deeply inwards (where 

 the contraction is very strong). The connective-tissue sub- 

 stance contains no elements for contraction ; the wandering 

 ceils occurring in it are easily distinguished by their nucleus 

 with nucleolus and their dissimilar contents from the contrac- 

 tile cells with uniformly granulated protoplasm and nucleus 

 with a network. Since besides these there are only spicules 

 with their cells and sexual products to be found in the 

 middle intermediate cell-mass, and as, further, the above- 

 mentioned ectoderm cells appear regularly in a form corre- 

 sponding to the contraction for the time being, it may be 

 rightly concluded that the seat of contractility in this simply 

 built sponge is still specially in the outer epithelial layer. 

 The simplicity of Leucosolenia clathrus is of course shown 

 also in the fact that it does not yet possess any separate 

 flagellated chambers, but that the whole internal cavity is 

 evenly clothed with collar-cells. The latter necessarily 

 take a passive share in the contraction, and then become 

 compressed in transverse diameter, corresponding to the 

 direction of contraction, and hence longer. 



Through the discoveries of Minchin as well as of Topsent 

 our attention is again drawn to the question referred to above, 

 raised by F. E. Schulze, as to whether sponges which show 

 three layers in the adult condition are not nevertheless merely 

 diploblastic animals (" Metamorphose von Sycandrarajjlianus^^ 

 Zeitschr. f. wiss. Zool. Band xxxi., 1878). The two recent 

 authors seek to arrive at a conception of the intermediate layer 

 by the histological method, since they look upon its elements 

 as not equivalent in themselves, but as standing in closer or 

 more distant relation to the primary layers. Topsent's merit 

 appears to me to consist in that he recognizes the contractile 

 cells of the intermediate mass as being much more similar 

 to the covering-cells than are the cells of the intermediate 

 mass among themselves ; of the latter there still remain 

 to him as specially mesodermal the cellules conjonclives (in 

 which the skeletogenous cells must also be included) and the 

 digestives pigmentees. Minchin also has attempted a similar 

 solution of the intermediate layer into its heterogeneous 



