MATERIALS FOB, A MONOGRAPH OF THE ASCONS. 535 



scleroblasts on the projecting raonaxous]. These, as well as 

 those of the inner surface, are retractile " — an observation 

 which, if true, would be of great interest.^ 



Not less remarkable than these early observations of Kolli- 

 ker and Lieberkiihn, which so nearly solved the question of 

 the origin of the gastral ray, is the fact that no further 

 observations upon the gastral rays are to be found in all the 

 voluminous literature of calcareous sponges until we come to 

 those of Dendy, to be mentioned in due course, published in 

 the present decade. 



The next author to formulate opinions upon the formation 

 of calcareous spicules was Haeckel, but his views were purely 

 theoretical and devoid of any basis of fact or observation. 

 Haeckel regarded the dermal layer of the sponge as a syn- 

 cytium, formed by the fusion of cells originally distinct in the 

 embryo. He supposed the ground substance or sarcodine to 

 be made up of the fused protoplasm of the cells ; and that in 

 this sarcodine, " at once the outer covering of the body and its 

 ' skeletogenous layer,' its contractile and its sensitive tissue " 

 (p. 164), the spicules arise by a sort of crystallisation, the 

 spicule sheath being formed by " a thickening and separation 

 from the sarcodine '^ (p. 167). Altogether a very clear and 

 logical theory, did it but harmonise with the facts. ^ 



Schulze, in his classical memoir on Sycon raphanus 

 (1875), did not make any observations on the origin of the 

 spicules, but appears to have been largely of Haeckel's opinion. 



^ Haeckel, having never seen a scleroblast — incredible though this may 

 seem, — understands Lieberkiihn to mean that the sheath is retractile, and 

 denies that this is the case. I do not attach this significance to Lieberkiihn's 

 statements. The scleroblast may quite well be retractile. 



^ It is remarkable that Haeckel should never have seen and figured the gastral 

 actinoblasts, especially after the clear descriptions of Kolliker and Lieberkiihn. 

 In such a figure as that of Ascaltis Gegenbauri (1872, pi. ix, fig. 7) we 

 are astonished to see nothing of these cells, usually so conspicuous. A closer 

 inspection, however, of the figure, to which reference has been made, reveals 

 a number of sperm masses in close proximity to the spicule ray, in such a way 

 as to provoke the suspicion that Haeckel confused two totally distinct kinds 

 of cell elements. 



