REPRODUCTION OF SPONGES. 121 



The most important fact in the life of a Sponge is that 

 which Robert Grant first observed, — that currents of water 

 pass gently in by the inhalent pores, and more forcibly 

 out by the exhalent aperture or apertures. This may be 

 demonstrated by adding powdered carmine to the water. 

 The instreaming currents of water bear dissolved air and 

 supplies of food, such as Infusorians, Diatoms, and particles 

 of organic debris. The outflowing current carries away 

 waste. When a sponge is fed with readily recognisable 

 substances, such as carmine or milk, and afterwards sec- 

 tioned, the grains or globules may Jae found {a) in the 

 collared endoderm cells ; (/;) in the adjacent phagocytes of 

 the mesoglcea ; (<:) in the phagocytes surrounding the sub- 

 dermal spaces, if these exist. It is uncertain whether the 

 epithelium of the subdermal spaces or the collared 

 endoderm is the more important area of absorption, 

 but it is certain that the phagocytes play an important 

 part in engulfing and transporting particles, in digesting 

 those which are useful, and in getting rid of the useless. 

 In an extract of several sponges, Krukenberg found a 

 (tryptic) digestive ferment, probably formed within the 

 phagocytes. 



Many sponges contain much pigment, thus the lipochrome 

 pigment (see Chap. XXIX.) zoonerythrin is common, and 

 like some others, such as floridine, is regarded as helping 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. 



Sponge growers often cut a sponge into pieces, and bed 

 these out in suitable places. The parts regenerate the 

 whole — a fact which illustrates the relatively undifferentiated 

 state of the sponge body. It is possible that fission may 

 also occur naturally. 



The frequent budding is merely a kind of continuous 

 growth, but when buds are set adrift, as sometimes happens, 

 we have discontinuous growth or asexual reproduction. 



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

 tion by statolilasts or gemmules. A number of mesogkeal cells occur 



