328 



THE BRITISH NATURE BOOK 



The shapes of the spicules help in many cases to differentiate the 

 species. 



This is only the barest and most general outline, and can only give a very 

 rough idea of the wonderful nature of these simple 

 animals. 



The two fresh-water Sponges referred to above 

 are the Eiver Sponge, found in the Thames and 

 other rivers and canals, as a dirty, yellowish slime 

 on wood and stone, and the Pond Sponge, which is 

 dark green, and found in rounded masses. 



During the whiter these die away, leaving cer- 

 tain cells which develop winter eggs and give rise 

 to male and female forms. 



Among the Marine forms the " Mermaid's Gloves " 

 (Chalina oculata) may often be picked up, and 

 recognized by its branching, tree-like shape, some- 

 times not unlike fingers. It may be from 6 to 10 

 inches long. It is a deep-water sponge, and is only 

 thrown up ashore after a storm. 



Another common form is the Crumb-of-Bread 

 Sponge (Halichondria panicea], found on large weeds 

 and covering rocks, looking when dry like a piece 

 of the crumb of brown bread. 



Amongst those that encrust the rocks and give 

 them their colour patches is the crimson-coloured 

 A, diagrammatic longitudinal Grantia coriacea, a thick soft substance, -1 inch in 

 section of a simple Sponge. The diameter. Another is Microciona carnosa pale red, 



P"** in thick narrow bands. 



Many oyster and scallop shells are found per- 

 coiiar-ceii; (in B), collar-cell ; f orated and bored with many small holes. These are 



P> P re; SpiC> the home of Cliona celata > a s P n S e w* 11 a yellowish 

 tinge, which it is supposed though the question is 



still unsettled has performed the work of drilling the holes, or dissolving 

 them away. 



Protozoa. 



These are the lowest forms of living animals, and are known popularly 

 as Animalcules. They can only be studied under the microscope. They 

 exist in countless myriads, and would require a whole volume to themselves. 

 They consist of single cells, and are so minute that hundreds can live hi a drop 

 of water. 



Many of them can be seen through the microscope described in the chapter 

 on Apparatus, and I know of no more entrancing study than that of the 

 contents of a drop of water thus inhabited. A delightful book on the subject 



collar-cell enlarged; col (in A), 



