THE SPONGES. 139 



have there called Archispongia, arose out of the Protascus 

 by the formation of pores through its body -wall; the 

 primary form of Sea-nettles, which I there called Archydra, 

 developed out of the Protascus by the formation of nettle- 

 organs, as also by the formation of feelers or tentacles. 



The main-class or branch of the Sponges, Spongice, or 

 Porifera, lives in the sea, with the single exception of the 

 green fresh-water Sponge (Spongilla). These animals were 

 long considered as plants, later as Protista; in most 

 Manuals they are still classed among the primaeval animals, 

 or Protozoa. But since I have demonstrated their develop- 

 ment out of the Gastrula, and the construction of their 

 bodies of two cellular germ-layers (as in all higher animals), 

 their close relationship to Sea-nettles, and especially to the 

 Hydrapolyps, seems finally to be established. The Olynthus 

 especiall}', which I consider as the common primary form of 

 calcareous sponges, has thrown a complete and unmistak- 

 able light upon this point. 



The numerous forms comprised in the class of Spongiae 

 have as yet been but little examined ; they may be divided 

 into three legions and eight orders. The first legion consists 

 of the soft, gelatinous Mucous Sponges (Myxospongise), 

 which are characterized by the absence of any hard 

 skeleton. Among them are, on the one hand, the long-since- 

 extinct primary forms of the whole class, the type of which 

 I consider to be the Archispongia ; on the other hand there 

 are the still living, gelatinous sponges, of which the Halisarca 

 is best known. We can obtain a notion of the Archispongia, 

 the most ancient primaeval sponge, if we imagine the 

 Olynthus (see Frontispiece), to be deprived of its radiating 

 calcareous spiculse. 



