ORDINARY FUNCJ^IONS. 129 



we see, as it were, huge hexactinellid spicules, spliced to- 

 gether with rope. It is convenient to distinguish the large 

 macroscleres from the small microscleres. Each spicule 

 begins to be formed by one or more " scleroblasts," and 

 may be speculatively regarded as an organised intra- 

 cellular excretion. " During its growth," Professor Sollas 

 says, " the spicule slowly passes from the interior to the 

 exterior of the sponge, and is finally (in at least some 

 sponges Geodia, Stelletta) cast out as an effete product." 

 The fibres of spongin are formed as the secretions of 

 mesogloea cells, known as spongioblasts. 



Ordinary functions. Excepting the fresh-water Spong- 

 illidse, all sponges are marine, occurring from between 

 tide marks to great depths. After embryonic life is 

 past, they live moored to rocks, shells, seaweeds, and 

 the like. Their motor activity is almost completely 

 restricted to the lashing movements of the flagella, the 

 migrations of the phagocytes, and the contraction of 

 muscular mesogloeal cells, especially around the exhalant 

 apertures. In the closure of the inhalant pores, sponges 

 show sensitiveness to injurious influences, but how far this 

 is localised in specialised cells is uncertain. 



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 inhalant pores, and more forcibly 

 out by the exhalant 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 

 sectioned, the grains or globules may be found (a) in the 

 collared flagellate cells ; (<) in the adjacent phagocytes of 

 the mesoglcea ; (c) in the phagocytes surrounding the sub- 

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

 epithelium of the subdermal spaces or the flagellate lining 

 of the deeper cavities is the more important area of absorp- 

 tion, 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. 



