and their lielationshij) to the Corals. 113 



The ectoderm of the Calcispongise, which becomes con- 

 verted by the fiision of the originally separate cells of the outer 

 or animal germ-lamella into the in some respects retromor- 

 jihosed tissue of the sarcodine or syncytium^ represents, physio- 

 logically considered, a tissue which performs the whole of the 

 animal functions of the sponge-body — movement^ sensation, 

 supjwi'tj and covering. The amalgamated protoplasm of the 

 sarcodine is contractile and sensitive, forms the skeleton, and 

 covers the surface of the body. It therefore, as it were, unites 

 in one person the four functions which, in the higher animals, 

 are separated and distributed over the four tissue-systems of 

 the muscles, nerves, skeletogenetic connective substances, 

 and epidermoidal covering. 



In a morphological point of view, of all the functions of the 

 ectoderm its skeletogenetic activity indisputably produces the 

 most imjjortant results. The skeleton of the Calcispongiae, as 

 indeed of all other sponges, is purely the product of the ecto- 

 derm — and, indeed, never a simple exudation, an " external 

 plasma-product," as I have expressed this idea in my ' Ge- 

 neral Morphology,' but always an internal plasma-product. 

 The qucestio vexata, so often ventilated, whether the skeletal 

 parts of the sponges are or are not produced in the interior of 

 cells, is solved by the developmental history. When the 

 skeletogenetic protoplasm still persists in the form of a distinct 

 cell provided with a nucleus, the spicules are produced in the 

 interior of this cell. But when the skeletogenetic cells have 

 already become fused together to form sarcodine, the skeletal 

 parts are produced in the interior of this syncytium. The ske- 

 letal parts of the sponges are never produced at the free surface 

 of the ectoderm, hut always in its interior. 



In the calcareous skeleton of the Calcispongias, by which 

 these sponges are distinguished from all others, we may with 

 comparative ease convince ourselves of this fact. The spi- 

 cules of the calcareous skeleton are in them either entirely 

 concealed in the modified protoplasm of the ectoderm, or, 

 when they project freely from its surface, they are still coated, 

 as if with a sheath, by a thin layer of the protoplasm. This 

 character, first indicated by Kolliker in Tarrus spongiosus 

 (his Nardoa spongiosa), has occurred to me more or less dis- 

 tinctly throughout the Calcispongiffi. Moreover in certain 

 cases the calcareous spicules contain a central canal filled 

 with protoplasm, such as occurs almost universally in the 

 siliceous spicules of the siliceous sponges. Lastly, in many 

 (perhaps in all?) Calcispongiae the carbonate of lime of the 

 skeleton appears not to be deposited quite pure, but to be in- 

 timately combined with a more or less considerable quantity 



