PORIFERA. 149 



it has no ancestral meaning but has been secondarily acquired ; but, assuming 

 that this is not the case, it appears to me that the characters of the larva 

 may be plausibly explained by regarding it as a transitional form between 

 the Protozoa and Metazoa. According to this view the larva is to be 

 considered 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. 



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 Spongida. 



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

 became fixed, the ciliated cells by which its movements 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 they were enabled to continue performing this 

 function through the formation of an osculum and pores. The collared cells 

 which line the ciliated chambers, 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 epiblastic 

 cells, which in most cases line the inhalent passages through its substance 1 , 

 ought to be employed to absorb nutriment. The recent researches of 

 Metschnikoff (No. 134) on this head shew that the nutriment is largely 

 carried into the mesoblast cells, which in Sycandra appear to be derived 



1 That the greater part of the flat cells which line the passages of most Sponges 

 are really derived from epiblastic invaginations appears to me to be proved by Schulze's 

 and Barrois' observations on the young fixed stages of Halisarca. Schulze's (No. 140) 

 observations have however proved that the flat cells lining the axial gastric chamber 

 of Sycandra are hypoblastic in origin, and the observations of Keller (No. 129) and 

 Ganin (No. 124) have led to the same result for the flat epithelium lining part of the 

 passages of the Silicispongise. 



