66 THE VICTORIAN NATURALIST. 



oxygen which is necessary for the continual oxidation of every 

 part of the sponge, but also particles of nutrient material which 

 will ultimately be absorbed by the special cells set apart for 

 digestive purposes. 



And this leads us to ask for a more detailed knowledge of the 

 system concerned in the alimentation of the sponge. Some of 

 the necessary details for this knowledge can be gained by 

 recourse to a few thin sections cut transversely with an ordinary 

 razor through one of the cylindrical sponges. If these sections 

 are placed in a shallow vessel the central gastral cavity is at once 

 apparent, and radiating from it towards the margin of the section 

 thimble-like spaces are seen, whilst between adjacent spaces is a 

 thin texture of gelatinous material in which are embedded 

 spicules of lime of various shapes and sizes matted together in a 

 definite manner to form a support to the spaces lying near to it. 

 The special arrangement of the spicules, like links in a suit of 

 chain-armour, to form the skeleton, allows, as we shall see after- 

 wards, of a free course to any water entering or leaving the 

 sponge. For the present we must leave the consideration of the 

 skeleton and confine our attention to the part played by the 

 thimble-like chambers in the very necessary duty of keeping the 

 sponge alive. Our rough sections will not furnish the necessary 

 details, but if by special methods of staining and embedding in 

 paraffin we are able to obtain sections the r ^ ¥ in. thick we see 

 at once that these chambers are lined by a very specialized kind 

 of cell, known from peculiar appendages it possesses as a "flag- 

 ellate collared cell," and giving to the chambers to which they are 

 confined the name of flagellate chambers. 



These collared cells play such an important part in the economy 

 of sponges that we must have a very clear conception of their 

 structure before we can enter, with intelligence, into the everyday- 

 life of such a sponge as we are now considering. They are 

 exceedingly minute in size, so small that from 5,000 to 10,000 of 

 them placed close together side by side would only form a line 

 one inch in length, and yet so wonderfully formed as to be able 

 not only to capture the food particles, but also to digest the same 

 in a manner analogous to the digestive cells of animals infinitely 

 removed from them in the animal kingdom. Typically, a collared 

 cell consists of a rounded or sometimes cylindrical body pro- 

 duced above into a neck. The neck is surmounted by a 

 comparatively long vibratile whip-like flagellum which is sur- 

 rounded by a very delicate transparent membranous collar which 

 is usually more or less funnel-shaped and inserted in the neck 

 around the flagellum. In the body of the cell is a specialized 

 part of the protoplasm, which shows a great affinity for the 

 staining material used in the preparation of the section, known as 

 the nucleus, and lying near to it are one or two open spaces called 



