SYCANDRA, 105 
are incommunication with each other towards their inner 
ends by minute cross-canals, sometimes called prosopyles. 
The thickened appearance of the exhalent canal-walls is 
due to the peculiar structure of the cells lining them. 
These are arranged in a single layer, and they consist of 
collared-flagellate cells, closely similar to those found in the 
choano-flagellate Protozoa. The currents of water bearing 
food-particles are due to the activity of these cells and their 
flagella. They are termed choanocytes. The outside surface 
ef the sponge is formed by flat irregular cells without 
flagella, which are known as finnacocytes. Similar pinnaco- 
cytes line the inhalent canals and the paragastric cavity. 
The whole limiting surfaces of the sponge are therefore 
formed either by a layer of pinnacocytés or of choanocytes. 
The space enclosed by the limiting surfaces seems to be 
filled with a semi-gelatinous matrix in which are numerous 
scattered cells. Most of these are branched or amceboid in 
appearance. Some surround and secrete spicules, one to 
each cell; these are the sclerocy¢es and they are said to 
periodically shed the spicules at the surface of the sponge. 
Others are in some way connected with nutrition, and yet 
others become ova and spermatozoa. These latter are the 
gonocytes or sexual cells, whilst the former are phagocytes. 
The alimentary processes of the sponge are not yet 
certainly known. Food-particles can be seen to pass in 
with the water at the pores and later the choano- 
cytes are crowded with them. Further, these~ 
food-particles may be seen in the phagocytes in the interior 
of the body. The choanocytes can withdraw their collars 
and flagella and become amceboid,* and it is questionable 
whether all the cells of the sponge are not capable on occa- 
sion of becoming amceboid, though this may not be normal. 
There are no definite excretory nor respiratory organs and 
no sense-organs nor nervous system. A few cells round 
some of the openings have been described as specially con- 
tractile and have been termed myocytes or muscle-cells. 
DEVELOPMENT.—Sycandra is dicecious, one sponge producing 
spermatozoa and another ova. The ovum is an amceboid gonocyte 
which protrudes into the lumen of an inhalent canal till it is fertilised 
Alimentary. 
* It is ‘more than probable” that the phagocytes are choanocytes which have 
changed to the amceboid condition and migrated inwards. 
