158 PHYLUM PORIFERA — SPONGES 



such as Pachymatisina ; skeleton-making cells ; pigment- 

 containing cells ; and lastly, the reproductive cells. 



{e) The skeleton consists of calcareous or siliceous 

 spicules, or of spongin fibres, or of combinations of the 

 two last. A calcareous spicule is formed of calcite, with a 

 slight sheath and core of organic matter ; a siliceous spicule 

 is formed of colloid silica or opal ; the spongin is chemi- 

 cally somewhat like silk. Uniradiate, biradiate, triradiate, 

 quadriradiate, sexradiate, and multiradiate spicules occur, 

 and they are eflfective in keeping the meshes open and in 

 giving the body architectural stability. In every pole 

 scaffolding we see, as it were, huge hexactinellid spicules, 

 spliced together 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 — Geodiiiy 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- 

 illidas, 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 ^i food, such as Infusorians, Diatoms, and particles 

 of organic debris. The outflowing current carries away 



