l60 PHYLUM PORIFERA — SPONGES 



workers regarding only whole organisms, and others 

 including colloidal particles, as solid food. There is good 

 evidence that even vertebrates can absorb and utilise 

 dissolved food-materials, if present in relatively high 

 concentration, but it is not clear that such absorption 

 takes place except in the alimentary canal ; in molluscs, 

 it appears to cease if the mouth is stopped up. Owing to the 

 difficulty of excluding bacteria, the experimental evidence 

 is not truly decisive ; and it is very doubtful whether 

 dissolved substances play an important part in nutrition. 



On a rather different level are the experiments of Peters, 

 who got various Infusoria to thrive on nutrient solutions, 

 apparently free from soUd particles or bacteria. Nitrogen 

 and carbon were present in the solution only in the re- 

 latively simple compound ammonium glycero-phosphate ; 

 rnost higher animals undoubtedly require more complex 

 nitrogen- and carbon-containing compounds (amino- 

 acids, at least). Further work on these lines would be 

 very valuable. 



Reproduction. — If a sponge be cut into pieces, these 

 may regenerate the whole — a fact which illustrates the 

 relatively undifferentiated state of the sponge body. It 

 is possible that fission may sometimes occur naturally. 



Ordinary budding is a mode of continuous growth, but 

 when small buds are set adrift, e.g. in Donatia and Tethya, 

 there is a form of asexual reproduction. 



In the fresh-water Spongilhdae there is a pecuUar mode of reproduc- 

 tion by statoblasts or gemmules. A number of mesogloeal cells occur 

 in a clump, some forming an internal mass, others a complex protective 

 capsule, with capstan-like spicules, known as amphidiscs. According 

 to W. Marshall, the life-histor\^ is as follows : In autumn the sponge 

 suffers from the cold and the scarcity of food, and dies away. But 

 throughout the moribund parent gemmules are formed. These survive 

 the winter, and in April or May they float away from the dead parent, 

 and develop into new sponges. Some become short-lived males, others 

 more stable females. The ova produced by the latter, and fertilised 

 by spermatozoa from the former, develop into a summer generation of 

 sponges, which, in turn, die away in autumn, and give rise to gemmules. 

 The Ufe-history thus illustrates what is called alternation of genera- 

 tions. Interpreted from a utihtarian point of view, the formation 

 of gemmules is a Hfe-saving expedient. As Professor SoUas says, 

 " the gemmules serve primarily a protective purpose, ensuring the 

 persistence of the race, while as a secondary function they serve for 

 dispersal." 



