980 HISTOLOGY AND NERVOUS SYSTEM OF CALCAREOUS SPONGES, 



those of other authors, point to the fact that the inner part, the 

 part produced first, of the spicule is softer and contains more 

 organic matter, whilst the outer layers, the youngest part, is harder 

 and resists the action of re-agents ; the whole spicule is composed 

 of prisms formed as cuticular productions by the cells clothing the 

 spicule from without. 



With colouring re-agents it is easy to prove the existence of a 

 highly colourable cuticle all over the spicule. On the part of the 

 spicules which protrude beyond the surface of the Sponge, the 

 cuticle is easily rubbed off, but always present, if the Sponge is 

 captured with sufficient care. The existence of a cuticle on the 

 protruding part can be proved without observation : were there 

 no cuticle the spicule would very soon be dissolved by the sea- 

 water and traces of a corrosive action thereof are never met with, 

 unless in those spicules found on the bottom of the sea or taken 

 up by other Sponges or Foraminifera to help to build up their 

 skeleton. 



This cuticle is slowly converted into spicule substance by an 

 interposition of molecules of carbonate of lime. This is the cause 

 of the lamella-structure of the spicules. Outside the Mesodermal 

 cuticle flat, Endothel-cells ai'e met with, covering the immersed 

 part of the spicule, in the shape of a hollow tube. Also on the 

 protruding part of the spicules such cells can be detected, but I 

 believe the latter to be Ectodermal, and nothing else than a 

 continuation of the Ectodermal pavement Epithelium. 



The spicules firstly make their appearance within cells, and the 

 axial rod (not canal !) is part thereof. The succeeding layers are 

 cuticular productions of Endothel cells. 



It is remarkable that the rays of the spicules are often curved 

 in various ways. 



THE MUSCULAR CELLS. 



Although all the tissue-cells, particularly in the Homocoela are 

 to be considered as contractile, still there are some Mesodermal 

 elements which aie developed in such a manner that their muscular 

 nature is much more expressed than in in the ordinary tissue cell. 



