130 PHYLUM PORIFERA SPONGE*. 



In an extract of several sponges, Krukenberg found a 

 (tryptic) digestive ferment, probably formed within the 

 phagocytes, but digestion is wholly intracellular. 



Many sponges contain much pigment; thus the lipo- 

 chrotne pigment zoonerythrin (familiar in lobsters) is 

 common. Some pigments, such as floridine, may help in 

 respiration. The green pigment of the fresh-water sponge 

 is closely analogous, if not identical, with chlorophyll, and 

 probably renders some measure of holophytic nutrition 

 possible. 



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, 

 we have a form of asexual reproduction. 



^ In the fresh-water Spongillidse there is a peculiar mode of reproduc- 

 tion by statoblasts or gemmules. A number of mesoglceal 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 history 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 life history thus illustrates what is called alternation of genera- 

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

 of gemmules is a life-saving expedient. As Professor Sollas says, 

 "the gemmules serve primarily a protective purpose, ensuring the 

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

 dispersal." 



All sponges produce sex cells, which seem to arise from 

 amoeboid mesogloea cells retaining an embryonic character. 

 In the case of the ovum, the amoeboid cell increases in 

 size, and passes into a resting stage; in the case of the 

 male elements, the amoeboid cell divides into a spherical 

 cluster of numerous minute spermatozoa. The similar 

 origin of the ova and spermatozoa is of interest. Most 

 sponges are unisexual, but many are hermaphrodite. In 



