and l/icir Ih'IalionaJiijj tu the Corals, 113 



The ectoderm of tlie CalcLspoiigia?, which becomes con- 

 verted by t\\c, fusion of the origiaallij .separate cells of the outer 

 or animal germ-himeUa into tlie in some respects retromor- 

 phosed tissue of the sarcodine or syacytiiDtij rejiresents, pliysio- 

 logieally considered, a tissue Avliieh performs the whole of the 

 animal functions of the sponge-body — moceinentj sensation^ 

 support, 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 

 tlie 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 important results. The skeleton of the Calcispongias, as 

 indeed of all other sponges, is piurelg 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 ^lorphology,' but always an internal plasma-product. 

 The qiuHstio vexata, so often ventilated, whether the skeletal 

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

 cjlls, 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 tlie skeletogenetic cells have 

 already become fused together to form sarcodine, the skeletal 

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

 letal 2)arts of the sponges are never produced at the free surface 

 of the ectoderm, hut always in its interior. 



In the calcareous skeleton of the Caleispongia?, 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 KoUiker in Tarrus sjwngiosus 

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

 tinctly throughout the Calcispongiffi. Moreover in certain 

 ca^e^ the calcareous spicules contain a central canal tilled 

 with protoplasm, such as occurs almost imiversally in the 

 siliceous spicules of the siliceous sponges. Lastly, in many 

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

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

 timately couibined with a more or less considerable quantity 



