24 STRUCTURE AND LIFE-HISTORY OF A SPONGE. 
having a nearly spherical body, flattened below where it is seated 
on the inner face of the mesoderm^ and rounded above and pro- 
longed into a long neck or collum. Near its end the collum is 
surrounded by a delicate collar of transparent sarcode^ which gives 
it the form of a wine glass. From the end of the collum is pro- 
duced a long filament of hyaline protoplasm, known as the 
flagellum, because it flagellates the surrounding water. 
The cell, in general, consists of a more fluid granular central 
protoplasm, or endosarc, surrounded by an outer firmer trans- 
parent contractile layer, or ectosarc. It is the ectosarc which is 
extended to form the collar and flagellum, which we may regard 
as highly specialised derivatives of pseudopodia. In the endosarc 
is a conical nucleus, surrounded by numerous granules, and some 
little blebs of watery fluid, known as vacuoles. Altogether the 
cell closely resembles some forms of infusoria, and it is of such 
cells, arranged close together, side by side, that the endoderm is 
wholly composed. 
Before describing the remaining tissues of the sponge, it will be 
convenient to introduce here a short account of its physiology, so 
far as the knowledge we have now attained of its structure makes 
possible. 
The flagella of the endodermic cells, which, as we have already 
noticed, form a continuous lining to the inner wall of the radial 
tubes, are almost always in motion, bending downwards with a 
rapid movement in one direction and then returning to their 
position of rest, and doing this so rapidly that the eye cannot 
follow them in the active state, so that usually they are quite 
invisible. Each movement of the flagellum ^ flicks ’ the water, as 
it were, in one direction, and the rapid successive movements of 
the almost infinitely numerous flagella drive the water out of the 
radial tubes into the stomach of the sponge, from which it emerges, 
to the ectoderm, though it appears to me that it might more naturally be 
regarded as endoderm. 
