228 THE ZOOLOGIST. 



medusce, Corals, and Ctenopliora are ectodermal or entodermal, 

 those of the Sponges are mesodermal. In the former the cells of 

 the epithelia are differentiated ; the epithelium of sponges is 

 alwajfs simple. The muscles, nerves, gland-cells, &c., which we 

 find in sponges are not modified epithelium cells, but differentiated 

 elements of the mesoderm. There is no subepithelium in sponges. 

 The same kinds of cells which we find in the Epithelaria, with the 

 exception of the cnidoblasts, are also found in sponges. But here 

 all cells which do not clothe the surface are of mesodermal origin. 



Movements in adult sponges were observed by Aristotle. 

 These movements are performed by cells which are called con- 

 tractile fibre-cells by F. E. Schulze. They are elongated, spindle- 

 shaped elements, which are mostly found around the pores. 

 I have also found such cells in other parts of the sponge, 

 and Sollas describes similar elements in the sphincters which 

 divide the subdermal cavities of the Geodidce from the inhalant 

 canals. 



The skeleton of the Sponges which belong to the order 

 Cornacuspongice appears as a network of fibres, which are com- 

 posed of series of spicules cemented together by spongin. Reniera, 

 Halichondria, &c., possess only a very small quantity of spongin. 

 In the Chalinince and Desmacidonidce the number and size of the 

 spicules decreases, and the spicules are replaced by the spongin- 

 cement more and more. In the Horny Sponges the siliceous 

 spicules have disappeared entirely from the supporting skeleton, 

 which consists exclusively of spongin — a substance identical with 

 the cement of the Halichondria skeleton. There may be foreign 

 bodies imbedded in the spongin-fibres. The spongin of different 

 Cornacuspongics is not always the same ; according to Eidley and 

 Vosmaer it difi'ers in diff'erent cases in its behaviour towards 

 polarised light. The colour of the spongin is very variable; 

 the cement of many Halichondridoz is hyaline, that of many Des- 

 macidomd(B and Spongidce light brown, of A2)lysillidce light orange 

 and sometimes black. The horny fibres of the Spongelidce appear 

 light yellow and transparent. 



The spongin of dry skeletons is, as a rule, darker than that of 

 living sponges or spirit specimens ; it is very tough and elastic. 

 According to Ivrukenberg, the chemical composition of spongin 

 is C3oH4eN90, s. and is thus similar to that of chitin, conchio- 

 lin, and cornein. Spongin is dissolved by boiling acids, and, 



