1903 



British Field Zoology 



34i 



under layers, whilst an amoeboid cell may come to the surface and 

 transform itself into a collar-cell. Whether this be so or not, one thing 

 is certain : that both classes of sponge-cell are distinctly pro- 1 

 tozoic in character. 1 



We will now examine the internal economy of the sponge 

 and the arrangement of its component parts. In Spongilla 

 these are rather complicated, so that it will be easier to 

 understand if a diagram is given rather than an exact repre- 

 sentation. Much detail is omitted, and the cells represented 

 larger in proportion than they really are. The sponge is a 

 mass of little tubes enlarged at intervals into rounded 

 chambers. These chambers are lined with the collar-cells just 

 described. The flagella of the collar-cells by their united 

 action cause a strong current to flow along the canals and 

 through the chambers. These currents always move in one 

 direction, from the exterior inwards. The canals open into 

 one another — the diameter increasing progressively — till they p- 

 finally all empty into one big tube, through which the united Monos'i'm 

 currents issue in one strong stream, as shown in Fig. 20. 

 It will be seen, then, that the cause of the current is to be found, not 

 in the big exit-tube, but in the small tubes which empty their contents 

 into it. 



The collar-cells take up food particles from the water as it passes over 

 them. They feed, 

 however, not only for 

 their own good, but 

 for the good of the 

 whole colony ; they 

 also, regarded as a 

 whole, elaborate the 

 flinty spicules which 

 stiffen the whole mass 

 and form its skeleton. 



Considered alone, ~) 

 each cell seems to be 

 a Protozoon, but col- 

 lectively, especially 

 when we note the 

 complex skeleton, we 

 are driven to the con- y\^ 2 o. — Diagram section of sponge showing chambei i 

 elusion that here is Lined with collar-cells ; also inhalant and exhalant 



seen a degree of apertures. 

 specialisation far be- 

 yond anything to be met with amongst the Protozoa. 



The difference becomes much more apparent when we come to 

 consider the reproductive processes in sponges, for in them we meet 

 with what are apparently true sex-cells, ova and spermatozoa. Both 

 these elements are produced in the same way. The cells which line 



