666 MORPHOLOGY AND SYSTEMATIC 



as a colony of Protozoa, one half of the individuals of which 

 have become differentiated into nutritive forms, and the other 

 half into locomotor and respiratory forms. The granular 

 amoeboid cells represent the nutritive forms, and the ciliated cells 

 represent the locomotor and respiratory forms. That the passage 

 from the Protozoa to the Metazoa may have been effected by 

 such a differentiation is not improbable on a priori grounds, and 

 fits in very well with the condition of the free swimming larva 

 of Spongida, though another and perhaps equally plausible 

 suggestion as to this passage has been put forward by my friend 

 Professor Lankester 1 . 



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 

 explanation 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 im- 

 probable that it was completely localised in them, but the 

 continuation of their function was provided for by the formation 



1 " Notes on P'mbryology and Classification." Quarterly Journal of Microscopical 

 Science, Vol. xvtl. 1877. It seems not impossible, if the speculations in this paper 

 have any foundation that while the views here put forward as to the 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. 



