108 F. M. BALFOUR. 



While the above view seems fairly satisfactory for the free 

 swimming stage of the larval Sponge there arises in the subsequent 

 development a difficulty which appears at first sight fatal to it. 

 This difficulty is the invagination of the ciliated cells instead of 

 the granular ones. If the granular cells represent the nutritive 

 individuals of the colony, they and not the ciliated cells ought 

 most certainly to give rise to the lining of the gastrula cavity, 

 according to the generally accepted views of the morphology of 

 the Spongida. The suggestion which I would venture to put 

 forward in explanation of this paradox involves a completely 

 new view of the nature and functions of the germinal layers of 

 adult Sponges. 



It is as follows : — When the free swimming ancestor of the 

 Spongida became fixed, the ciliated cells by which its move- 

 ments used to be effected must have to a great extent become 

 functionless. At the same time the amoeboid nutritive cells 

 would need to expose as large a surface as possible. In these 

 two considerations there may, perhaps, be found a sufficient expla- 

 nation of the invagination of the ciliated cells, and the growth 

 of the amoeboid cells over them. Though respiration was, no 

 doubt, mainly effected by the ciliated cells, it is improbable that 

 it was completely localized in them, but the continuation of their 

 function vras provided for by the formation of an osculum and 

 pores. The ciliated collared cells which line the ciliated cham- 

 bers, or in some cases the radial tubes, are undoubtedly derived 

 from the invaginated cells, and if there is any truth in the above 

 suggestion, the collared cells in the adult Sponge must be mainly 

 respiratory and not digestive in function, while the normal epi- 

 thelial cells which cover the surface of the sponge, and in most 

 cases line the greater part of the passages through its substance, 

 must carry on the digestion.^ If the reverse is the case the 

 whole theory falls to the ground. It has not, so far as I know, 

 been definitely made out where the digestion is carried on. 

 Lieberkiihn would appear to hold the view that the amoeboid 

 lining cells of the passages a^re mainly concerned with digestion, 

 while Carter holds that digestion is carried on by the collared 

 cells of the ciliated chambers. 



1877. It seems not impossible, if the speculations in this paper have any 

 foundation that while the views here put forward as to tlie passage from the 

 Protozoon to the Metazoon condition may hold true for the Spongida, some 

 other mode of passage may have taken place in the case of the other 

 Metazoa. 



* That the flat cells which line the greater part of tlie passages of most 

 Sponges are really derived from ectodermic invaginations appears to me 

 clearly proved by Schulze's aud Barrois' observations on the yout)g fixed 

 stages of Halisarca. Gauin appears, however, to maiutain a contrary 

 view for Spougilla. 



