SYCANDRA. 105 



are in communication with each other towards their inner 

 ends by minute cross-canals, sometimes Q,2\\^di 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 

 of the sponge is formed by flat irregular cells without 

 flagella, which are known as pinnacocytes. Similar pinnaco- 

 cytes line the inhalent canals and the paragastric cavity. 

 The whole limiting surfaces of the sponge are therefore 

 formed either l^y a layer of pinnacocytes 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 amoeboid in 

 appearance. Some surround and secrete spicules, one to 

 each cell ; these are the sderocytes 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 

 Alim ntar ^^^^ ^^ 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 amoeboid, 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-ctlls. 



Development. — Sycandra is dioecious, one sponge producing 

 spermatozoa and another ova. The ovum is an amoeboid gonocyte 

 which protrudes into the hnnenofan inhalent canal till it is fertilised 



* It is "more than probable" that the phagocytes are choanocytes which have 

 changed to the amoeboid condition and migrated inwards. 



