258 Dr. Gr. C. J. Vosmaer on the 



little avail to seek to explain how a sponge-larva or a primi- 

 tive sponge has arisen from a colony of Protozoa. Balfour's 

 theory is based upon pure assumption, and so is Heider's. 

 It would be just as possible that, after the functional differen- 

 tiation had taken place in the cells of a colony of Protozoa, the 

 larva became, owing to the formation of spicules, too heavy 

 to swim and sank to the ground, wherein lies a great incentive 

 to become attached. The early, often very early, appearance 

 of the spicules may be urged in favour of this view. But all 

 this, as we have said, is as yet pure hypothesis, for which 

 certainly much may be adduced ; but it appears to me still 

 rather purposeless to philosophize much about the matter. 



If we accept a free-swimming form as the ancestor, and 

 suppose further that solid structures became secreted in cer- 

 tain cells (thereby conferring an advantage in rendering these 

 delicate forms of life less subject to fall a prey to other ani- 

 mals), then we must at the same time believe that in one 

 group calcareous and in another siliceous matter was deve- 

 loped. But this new development led to the restriction, nay 

 finally even to the complete prevention, of free movement, and 

 thereby a higher animal development was precluded. Sessile 

 animals must develop in a special direction in order to main- 

 tain the struggle for existence. Nutrition and respiration must 

 be assured ; hence, though the degree of development is a low 

 one, yet a well-developed canal-system has been formed. 



A second supposition to which we are forced is that Sponges 

 originally lived in tolerably great depths. The oldest forms 

 are, emphatically, deep-sea forms. When, at a later date, 

 they also lived in shallow regions, we see in arrested develop- 

 ment the consequences of such a proceeding. The whole class 

 of Porifera noa-calcarea appears to indicate this. First the 

 skeleton degenerates^ the relative amount of silica decreases, 

 and the variety of spicular forms is step by step reduced. At 

 the same time the independent characteristic form is lost ; 

 but in certain examples the canal-system develops progres- 

 sively, not in constant, although probably direct, or inverse 

 relation to the skeletal system. 



Thus from the primitive form have arisen, in the first place, 

 the Calcareous Sponges, a group in which the canal-system is 

 most complex in those forms which show degeneration in the 

 skeleton. From the primitive form of the Calcarea, perhaps an 

 Olyntlius-\^k& sponge, arose, on the one hand, the Asconidce^ 

 and, on the other, the ancestors of the Sycons, from which the 

 Syconidai of the present day have been developed ; but also, 

 as we have fairly good reasons for believing from Poldjaeff's 

 researches, the Leuconidoi and Teichonidce. The position of 

 the Fharetronidce remains doubtful. 



