1892.] 



Note on Excretion in Sponges. 



481 



abandonment of the old view, that the horny fibres are derived from 

 the cnticle and the ectoderm ; I can see no evidence advanced for 

 the mesodermal nature of the " cap-like investment belonging to 

 the dermal layer of the connective tissue " which Schulze finds 

 (' Spongidee,' 1879, p. 637*) to be the organ productive of the 

 principal growth of the fibre ; I think that Lendenfeld is probably 

 right in his homology of the spongoblasts with gland cells (1883, 

 ' Zeitschr. wiss. Zool.,' vol. 38, p. 256), and in his statement that 

 the two tissues are continuous one with another (' Horny Sponges,' 

 1889, p. 790). Having found with certainty that the flat epithelium, 

 which he draws and describes in detail (' Monograph of Australian 

 Sponges,' 1884, 1885, passim; 'Monograph of Horny Sponges,' 

 1889, p. 775; ' Kalkschwamme,' 1891, p. 162), over the gland cells 

 of calcareous sponges has no existence, I reject entirely his evi- 

 dence that a similar epithelium covers the gland cells of Ceratosa ; 

 the more so as Schulze, Polejaeff, and other conscientious workers 

 have failed to demonstrate it. I hold that these gland cells are the 

 ordinary flask-shaped epithelium which forms the external covering 

 of most sponges, and is presumably the ectoderm. I hold it on the 

 present evidence probable that the spongoblasts are homologous with 

 them, the horny fibre with the horny cuticle, and the hexagonal 

 markings of the horny fibre with the well known " silver lines " of 

 the outer surface. 



It is impossible for any unskilled person to attempt the chemical 

 comparison of such substances ; the more so as " spongin " is a body 

 — or a series of bodies — hitherto little investigated and vaguely de- 

 fined. As used by zoologists, it is a convenient word applied to a 

 number of different nitrogenous substances, not easily soluble, found 

 as supporting structure in a large group of more or less allied sponges ; 

 while the skeletons of some genera are easily soluble in caustic 

 potash (Kolliker), and even very dilute caustic potash (Dybowski), 

 in others they withstand it at boiling temperature, and in any 

 strength. f We have assumed these substances to be related because 

 of their homologous occurrence in allied animals ; I wish to extend 

 this assumption, for what it is worth, to the excretory granules of 

 Calcarea. It is noticeable that, like " spongin," the matter compos- 

 ing these granules is characterised by a tendency to great variation. 



* It is just to our leader to say that in the same paper lie expressly states that 

 he speaks of " connective tissue," because its homology with "mesoderm" is only 

 probable, and not proved (p. 625). But I suggest that, from his description, it is 

 open to the reader to believe that the cap belongs to the " outer cell-layer, and not 

 to the connective tissue layer." Compare especially p. 626. 



f Vosmaer points out that analyses have hitherto only been made on Euspongia, 

 The observations of Eidley, to which Yosmaer gives his support, show that the 

 horny skeletons of some Ceratosa are not doubly refractive. 



