496 CHARACTERS OF INVERTEBRATE ANIMALS 



together, or, and usually, of calcareous nature. It either con- 

 sists of a single chamber or of a number of chambers associated 

 in various manners and presenting the utmost variety in form. 

 One of the most abundant forms is Globigerina, the shell of 

 which consists of several rounded chambers united together, 

 and which occurs in such profusion in a calcareous deposit 

 covering large tracts of the ocean floor that this has received 

 the name of Globigerina ooze. Globigerina and other forms 

 have also lived in earlier epochs of the earth's history, playing 

 a very important part as rock-builders. The pure form of lime- 

 stone called chalk is, for example, mainly composed of such 

 remains. Most of the Foraminifera are of small size, but 

 Nummulina, so called from its coin-like shape (L. nummus, a 

 coin), reaches the size of a shilling piece. Its shells chiefly 

 make up the important nummulitic limestone by which mountain 

 ranges in the Mediterranean region are to a large extent built up. 



The pseudopods of the Foraminifera are slender threads which 

 often unite together into a sort of net-work. More than one 

 nucleus is commonly present in the protoplasm of which the soft 

 body consists. 



The Sun- Animalcules (Heliozoa) (fig. 301) are a group of 

 mainly fresh-water forms in which the body is spherical, and 

 stiff pointed pseudopods radiate from it in a way which suggests 

 the rays seen in the conventional representation of the sun; 

 hence the name. There may be more than one nucleus and 

 pulsating vacuole. Actinophrys and Actinosphcerium are typical 

 genera. In some members of the group the surface of the 

 body is covered by loose flinty spicules, and in the stalked 

 genus Clathrulina there is a continuous shell in the form of a 

 perforated hollow sphere. This leads on to the condition found 

 in most of the marine group of Ray -Animalcules (Radiolaria) 

 (fig. 301), where there is a siliceous shell of extreme beauty 

 and of the most varied shape. Some parts of the sea-floor are 

 covered by Radiolarian ooze, mainly composed of such shells; 

 and there are also fossil forms making up certain rocks, notably 

 in the Barbados. Almost every microscopic cabinet contains 

 a slide of this " Barbados earth ". 



The Amoeba group may be reckoned to include certain pro- 

 blematic organisms called Myxomycetes or Mycetozoa, the latter 

 name indicating the doubt which has existed as to whether 



