1 82 NATURE AND AFFINITIES OF THE SPONGES. 



It is, of course, possible that it has no ancestral meaning, but has been secondarily 

 acquired. I prefer myself to think that this is not the case, more especially as 

 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 d, priori grounds, and 

 fits in very well with the condition of the free-swimming larva of the Spongida. 



" While the above view seems f,iirly satisfactory for the free-swimming stage of 

 the larval sponge, there arises in the subsequent development a difficulty which at 

 first sight seems fatal to it. This difficulty is the invagination of the ciliated cells 

 instead of the granular ones. If the granular ones 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 the adult sponges. 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 j at the same time the 

 amceboid 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 impro- 

 bable that it was completely localized in them ; but the continuation of their function 

 was provided for by the formation of an osculum and pores. The ciliated collared 

 cells which line the ciliated chambers, or in some cases tiae 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 epithelial 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. Lieberkuhn would appear to hold the view 

 that the amoeboid lining cells of the passages are mainly concerned with digestion, 

 while Carter holds that digestion is carried on by the collared cells of the ciliated 

 chambers. If it is eventually proved by actual experiments in the nutrition of 

 sponges that digestion is carried on by the general cells lining the passages, and not 

 by the ciliated cells, it is clear that neither the ectoderm nor entoderm of sponges 

 will correspond with the similarly named layers in the Coelenterata and the Metazoa. 

 The invaginated entoderm will be the respiratory layer, and the ectoderm the diges- 

 tive and sensory layer ; the sensory function being probably mainly localized in the 

 epithelium on the surface, and the digestive one in the epithelium lining the passages. 

 Such a fundamental difference in the germinal layers between the Spongida and the 

 other Metazoa would necessarily involve the creation of a special division of the 

 Metazoa for the reception of the former group." 



It is very clear, after a perusal of Mr. Balfour's views as above 

 enunciated, that the claims of the Spongida for admission into the ranks of 

 the typical Metazoa, and more especially for correlation with the Coelen- 

 terata, as advocated by Professor Haeckel, rests upon the most shallow and 

 insecure foundation. It is likewise evident that in Mr. Balfour's estimation 

 the sponges retain a close kinship with the Protozoa, and are at the outside 



