INTRODUCTION. 



Thus, when all these monographs have appeared, our knowledge of Spouges 

 will be established on a hrm basis, as the only remaining group, the 

 Calcareous Sponges, has already been made the subject of a monograph by 

 Haeckel, and has been studied by Polejaeff and myself. 



This satisfactory and complete working out of Sponges is partly due to the 

 facilities afforded me by the Trustees of the Australian Museum and by 

 individuals in Xew South Wales. 



I have to perform the agreeable duty of expressing my thanks to the Hon. 

 William Macleay, who, in the first place, enabled me to study the rich Sponge- 

 fauna of Port Jackson carefully ; and to the Trustees of the Australian 

 Museum, and to Dr. Ramsay, the Curator, who have met and assisted me in 

 a liberal manner in carrpng on my researches. I am equally indebted to 

 the Authorities of the British Museum, and particularly to Dr. Giiuther and 

 Mr. S. O. Eidley, for the uniform kindness with which they assisted me 

 during the time J was working there. 



We may now review our knowledge of Sponges, and give an outline of the 

 chief results attained. 



The simplest Sponges (the Asconidse among the Calcarea) have the shape 

 of a sac the thin walls of which are perforated by numerous small holes. 

 This wall is composed of a structureless jelly- or glue-like substance known as 

 the ground-substance, in which cells of various kinds and also the skeleton are 

 contained. The surface of the mesogloea is covered with a continuous layer of 

 epithelial cells. Nowhere does the mesogloea come in direct contact with the 

 surrounding water. Every epithelium-cell bears one cilium, which is appa- 

 rently in continuous motion. The cells on the external surface are low and 

 flat, and derived from the ectoderm of the gastrula-larva ; those on the inner 

 side are high and cylindrical, and their free end bears a long flagellum and a 

 conic prolongation of the margin— a collar. These cells are of entodermal 

 origin, and known as collar -cells; they are very peculiar in shape, and no cells 

 of this kind have been found in any Metazoa except Sponges. They are 

 similar to certain flagellate Protozoa, Sa^nnr/ceca and Codosiga. 



The sexual cells, the connective-tissue cells, the muscular and sensitive cells, 

 the gland-cells, and the skeleton-producing cells are imbedded in the mesogloea, 

 which extends between the ectoderm and entoderm, and all these cells are of 

 mesodermal origin. 



The epithelia, both ectodermal and entodermal, invariably appear as single 

 layers of cells, and do not produce any organs of the interior. 



As mentioned above, the flagella are in continuous motion, aiid, particularly 

 those of the collar-cells, produce a pretty strong curi*ent of \\ater, which 

 enters by the small pores in the sac-wall and leaves by the large terminal 

 aperture — the mouth of the sac, which is termed the osculum. 



All the more highly organized forms of Sponges can be easily derived from 

 the simple Ascon described above. In the first place, the wall of the sac becomes 

 folded and forms diverticula. Then the collar-cells, which clothed the \\ hole of 



