Relationships of the Porifera, 255 



possibility that what is true for one division of the Sponges 

 is certainly false for another. It seems to me, and every- 

 thing* points to this conclnsion, that most siliceons sponges 

 are degenerating in a certain respect, but that in the Cornacu- 

 spongia3 a new force has stepped in which again lifts them 

 up, and that the Calcarea of the present day are also developing 

 progressively. But even if most Sponges do show numerous 

 traces of degeneration, yet they need not on that account be 

 descended from Coslcntcrates. Tiic ditferences between the 

 two groups are so great that even the most zealous advocate 

 of their coelenterate nature, as Ave have seen, puts their pliylo- 

 genetic connexion a very long way back ; and, in spite of 

 this, Marshall's theory is scarcely tenable. Granted that the 

 nearest ancestors of the Sponges were " at least two-layered," 

 granted also that they were " radiate," even that they pos- 

 sessed a '' gastral cavity " {s, L), &c., yet this shows nothing. 

 Such creatures are still not Ooelenterates. ]\rarshall, to be 

 sure, goes further, and claims for the sponge-ancestor a 

 " mouth-opening '' and a " gastral cavity " with centrifugal 

 canals ; but there are no grounds for this. For, as Heider 

 again asserts, the so-called osculnm of the Sponges is neither 

 homologous nor analogous with the mouth of Ccelenterates, 

 and the large internal cavity present in many Porifera has 

 just as little claim to the signihcance of a gastral cavity as, 

 in short, the canals in connection with it have to be placed on 

 the same footing as the peripheral canals of the Coelenterata. 

 There is not a single reason for regarding the central cavity 

 in Sponges as a gastral cavity. Even supposing that its 

 epithelium may, perhaps, take up nutrient particles, still it 

 has never yet been observed that the cavity is the true 

 digestive cavity, kut^ ^^o^^jv *. This is, moreover, very 

 improbable for several reasons ; for, in the tirst place, this 

 momentous cavity is not always present, or it is very small ; 

 and, in the second place, its position and arrangement are 

 very unfavourable for the retention of solid bodies. It may 

 be answered, that it has not yet been demonstrated that 

 proper solid nutriment is taken in. Since, however, it is 

 certain that particular sponge-cells aiii take in solid bodies, 

 and do so very readily, and, further, that sponges placed in 

 reservoirs which are kept as clean as possible, and where the 

 inllowing water is freed from suspended particles, perish more 

 rapidly than others which are kept in dirty {ttif renid verba) 

 reservoirs, it is, on this ground alone, more probable that solid 

 nutriment is a vital question with them. The unfavourable 

 nature of the position of the so-called gastral cavity depends 

 • Hackel's assertions i-est upou piu'e imagiuation. 



