Mr. D. Sharp on the Atlantic Species 0/ Philhydrus. 13 



of which communicates with the surrounding sea-water only 

 by a single aperture, the mouth, placed opposite to the point 

 of attachment. In this early young state, when it constitutes 

 a simple cup-shaped Uody with solid walls and a simple aper- 

 ture, the young sponge is not essentially different from a young 

 coral which is still in the same early period of ontogenesis. 

 But just as the common freshwater Polype [Hydra) presents 

 persistently throughout life, in its simple sac-like body-cavity, 

 a similar coelenteric primitive state to that which all corals pass 

 through in their youth, so does this just-mentioned simplest 

 calcareous sponge [Prosycum) remain throughout its life, until 

 perfect maturity, in the same coelenteric primitive state which 

 the other calcareous sponges have to pass through rapidly in 

 their earliest youth. Considering, now, that extremely impor- 

 tant and intimate causal connexion which everywhere exists 

 between ontogeny and phylogeny, — considering the morj)hoge- 

 netic fundamental law, that the ontogeny (that is to say, the 

 individual developmental history of the organism) constitutes 

 a short and rapid (causally conditioned by the laws of inherit- 

 ance and adaptation) repetition of its phylogeny , that is, of the 

 palffiontological developmental history of the ancestors of its 

 entire stock, — considering this high phylogenetic signification 

 of all ontogenetic states, we must^ from these simple facts, from 

 this ontogenetic concordance between the young states of the 

 sponges and corals, draw the extremely important phylogenetic 

 conclusion, that the sponges and corals are near hlood-re- 

 lations, whose origin is derived from one and the same ori- 

 ginal common stock-form. This unknown stock-form, of 

 whose special structure no fossil remains are preserved to us 

 from the archolithic period of the earth's history, but as to 

 whose former existence we may conclude with perfect cer- 

 tainty from the adduced facts, nay, of whose general form we 

 have even still an approximate picture in Prosycum simplicis- 

 simum !, must have possessed a simple cup-shaped body, with 

 a single orifice })laced opposite to its point of attachment. We 

 will give this the name of the primitive sac, Pkotascus. 

 From this hypothetical Protascus probably originated, as two 

 divergent branchlets, Prosycum (the stock-form of the Calci- 

 spongiee) and Procorcdlwn (the stock-form of the corals). 

 [To be continued.] 



II. — On the Species of the Genus Philhydrus_/o?/nc7 iri the 

 Atlantic Islands. By D. Sharp, M.B. 



When engaged last spring in making an examination of our 

 British Philhydri^ and comparing them with the few speci- 



