470 E. A. MINCHIN. 



Preface. 



The Ascons are a group of calcareous sponges which pre- 

 sent many points of interest and importance to the zoologist. 

 As sponges they occupy the lowest systematic position, for 

 though the possession of a distinct skeleton is an advance 

 upon the condition found in the so-called Myxospongise — sup- 

 posing these not to be degenerate in this respect, — yet in 

 almost all other points of organisation the Ascons show a far 

 more generalised condition than is found in any other class of 

 sponges. Regarded also not merely as sponges, but from the 

 wider standpoint of the Metazoa generally, the Ascons claim 

 our attention as extremely primitive types, characterised by a 

 simplicity of structure scarcely equalled in any other form of 

 animal life above the Protozoa. Not only, therefore, are 

 many problems of sponge morphology and embryology to be 

 found reduced to their lowest terms in the Ascon type, but it 

 might be expected further that a careful study of the group 

 would shed much light upon biological questions of still wider 

 significance, since many facts of organisation and development 

 which, in other animals, lie deep and are difficult to approach, 

 are in these forms almost upon the surface, as it were. 



For these reasons I have devoted myself for some time past 

 to studies upon the structure, development, and classification 

 of the Ascons, in the hope that a complete knowledge, so far 

 as this is possible, of the group might form a material contri- 

 bution to zoology. It might, however, appear to some that 

 there would be but little of importance to discover in the 

 group which was not already known, seeing how voluminous 

 is the literature of the Ascons, and how many authors have 

 published studies and monographs upon them even in quite 

 recent times. So far is the subject from being exhausted, 

 however, that the most elementary facts in their organisation 

 still remain unknown or misrepresented, and it is not too much 

 to say that the Ascons offer a field which, so far as results go, 

 is almost unexplored. If this statement seems to many 

 exaggerated, I can only hope to justify it by bringing forward 



