105 



acter of the organism is revealed. The bod}'- wall becomes per- 

 forated with pores, which open into the general cavity of the body, 



flow out through the so-called mouth. This is the " Proto- 

 spongia " state, and when spicules of silex or lime are developed 

 to strengthen the walls of the body, the young sponge is termed 

 by Haeckel, the « Olynthus." 



Thus following the cou--e of development as Haeckel supposed 

 to be the case with the calcareous sponges, for he, as Metschnikoff 

 remarks, did not actually observe the stages after the formation 

 of the ciliated larva we obtain ^ a very clear idea of the typi- 



condensed account of the discoveries of Haeckel given by Dr. 

 Liitken in the "Zoological Record" for 1872, with a few correc- 

 tions taken from MetschnikofFs paper. The Olynthus, the sim- 

 plest type of the sponge, is a " cylindrical, clavate or pyriform, 

 etc., tube, closed at the extremity by which it is affixed, commonly 

 open by a ' mouth ' at the other ; the body-wall, enclosing the 

 'gastric' cavity, is a thin membrane composed of the two layers 

 named above— the 'syncytium' or exoderm [Metschnikoff's inner 

 layer] a mass of sarcodine with nuclei, the cells of which are so 

 completely fused together that the original cellular structure 

 cannot be made visible through any chemical reaction ; if torn 

 mechanically, the fragments will, whether containing one or more 

 or no nuclei, take the shape of Amoebaa and walk about. In this 

 layer 8 the spicula are developed, chiefly of three types — simple, 

 3-radiate and 4-radiate, anchor-shaped spicula are rare (Syculmis 

 synapta, anchoring the animal in the mud bottom) ; the stellate 

 spicula sometimes occurring are foreign bodies, belonging origin- 

 ally to Didemnia (Ascidia?). The spicula are invested with a 

 delicate sheath of condensed sarcodine ; they contain an axial 



cious spicula ; chemically they are composed partly of Co 2 , CaO, 

 partly of an organic substance ('spiculin'). The endodermal 

 cells are, like certain flagellate Infusoria, provided with a collar 

 and flagellum; they contain a 'nucleus' (with 'nucleolus'), 

 and often one or two contractile 'vacuola' (water drops); 

 though without 'mouth,' they both 'drink' and 'eat,' or receive 



