XII 



BACKBONELESS ANIMALS 



225 



ally flow through the sponge. The sponge is a Venice - 

 like city of cells, penetrated by canals, in which incoming 

 and outflowing currents are kept up by the lashing activity 

 of internal flagellate cells. These flagellate cells, on which 

 the whole life of the sponge depends, line the canal-system, 

 but are especially developed in little clusters or flagellate 

 chambers. The currents are drawn in through very 

 small pores all over the surface ; they 

 usually flow out through much larger 

 crater-like openings. 



Sponges feed easily and well, and 

 many of them grow out in buds and 



FIG. 63. AN ENORMOUSLY ENLARGED REPRE- 

 SENTATION OF A FLAGELLATE INFUSORIAN, 

 ONE OF THE CHOANOFLAGELLATA, OFTEN 

 CALLED A MONAD. 



(After Saville Kent.) 



The animal is fixed to some object by a stalk 

 (ST). It is surrounded by a cuticular cup (CA). 

 The living matter is extended into a beautiful 

 delicate collar (C), in the middle of which there 

 works a vibratile flagellum (FL). 



In the cell-substance or cytoplasm a nucleus (A 7 ) 

 is shown, and there are granules and vacuoles (V). 



This kind of collared flagellate cells is found 

 only in Choanoflagellata among Infusorians and 

 in the interior of sponges. This suggests that 

 sponges may have been evolved from colonies of 

 Choanoflagellates. The collared cells of sponges 

 are called choanocytes, and it is their activity 

 chiefly that keeps the water-currents a-going. 



branches. A form which was at first 



a simple cup may grow into a broad 



disc or into a tree-like system. And 



as trees are blown out of shape by 



the wind, so sponges are moulded by 



the currents which play around them, 



as well as by the nature of the objects on which they 



are fixed. Like many other passive organisms, sponges 



almost always have a well-developed skeleton, made 



of flinty needles and threads, of spicules of lime, or 



of fibres of flexible spongin familiar to us in the bath 



sponge. While sponges do not rise high in organic 



rank, they have many internal complications and much 



beauty. 



16 



ST. 



