SPONGES 161 



in other directions. The former remained at the surface, the latter tended 

 to migrate into the interior. 



Next followed the very obscure portion of the phylogenetic history in 

 which the ancestor became fixed, and underwent changes which resulted 

 in the nutritive collar cells becoming placed in the interior to form the 

 gastral layer, while the other cells came to surround them and form the 

 dermal layer. Although these two events, the fixation and the reversal 

 of the layers, doubtless stand in close relation to one another, it is difficult 

 to say which preceded the other, or to attempt to follow this period of the 

 evolution in detail. It is represented in ontogeny by the metamorphosis, 

 which, like all similar stages throughout the animal kingdom, evidently 

 represents a large and important series of phylogenetic stages compressed 

 into a very short time, and much modified in nature. When once the 

 metamorphosis is past, the subsequent pupal stages are not difficult to 

 interpret. It has already been pointed out that the flattened pupa formed by 

 metamorphosis of the larva of Clathrina may be regarded as a very simple 

 type of sponge in a state of extreme contraction. Its further histogenetic 

 development, which in ontogeny takes place in the contracted condition, 

 or during the gradual process of expansion, gives us a clue to understand- 

 ing how in phylogeny the calcareous Olynthus was evolved from the simple 

 Protolynthus, the ancestor of all sponges. 



The Choanoflagellate theory of sponge ancestry may be said therefore 

 to afford a simple interpretation of all stages of the embryology, with the 

 exception of the metamorphosis, a portion of their life-history of which the 

 significance still remains very obscure. We may console ourselves, how- 

 ever, with the thought that the metamorphosis is equally, if not more, 

 difficult to interpret from a phylogenetic point of view on the Enterozoic 

 theory, and that it becomes absolutely unintelligible from any point of 

 view if the Coelenterate theory be adopted. 



(b) The Phylogeny of Sponges. Three main lines of descent and 

 evolution can be recognised in sponges generally, represented by the 

 Calcarea, the Hexactinellida, and the Demospongiae respectively. 

 In the former we have a very distinct stock, with no transitions to 

 other forms. The Hexactinellids, on the other hand, have in the 

 siliceous nature of their skeleton a feature which links them with 

 the Demospongiae, but it is open to discussion whether this peculi- 

 arity is inherited by both from a common ancestor with a siliceous 

 skeleton, or has been independently acquired. 



Haeckel believes the common ancestor of all sponges to have been a 

 form which inhabited the deep sea, and was provided with a pseudo- 

 skeleton of foreign bodies, consisting chiefly of the skeletons of Radiolaria, 

 Foraminifera, and other pelagic animals which were continually showering 

 down upon it. This primitive sponge next acquired the power of dissolv- 

 ing the siliceous and calcareous matter which it took up, and depositing 

 the mineral substance anew in the form of spicules in the tissues. Some 

 sponges, which lived in Globigerina ooze, acquired in this way a calcareous 

 skeleton ; others living in Radiolarian ooze acquired a siliceous skeleton. 



