EHIZOPODA. 25 



procesaea of sarcode, which can be thrust out at will, and which melt 

 again into the substance of the body when they are withdrawn. 



Four principal types of structure are known in the Rkizopoda, and 

 these constit\ite as many distinct orders, which are known by the 

 names of the Monera, Amcebea, Foraminifera, and Radiolaria. 



Order I. Monera. 



This name has been proposed for a small group of organisms 

 which are not only the lowest types of the Ehizopods, but may be 

 regarded as the most elementary forms of animal life known to 

 us. The Monera are minute protoplasmic organisms in which the 

 sarcode-body is neither defended by a thickened external envelope, 

 nor furnished with the structures known as the " nucleus " and 

 "contractile vesicle." Vacuoles and granules are present in the 

 sarcode, and the pseudopodia emitted by the soft and mutable pro- 

 toplasm of the body are delicate and thread-like, or may be ramified 

 and variously interlaced (fig. 5). No hard covering or shell is ever 

 developed. The Moners inhabit water, and are mostly marine. 



Order II. Amcebea. 



This order is characterised by the fact that the pseudopodia are 

 mostly blunt and finger-like in shape, and that the sarcode of the 

 body contains the structures known as the " nucleus " and " contractile 

 vesicle." 



As the type of the order may be taken the Amoeba or Proteus- 

 animalcule, so called because of the incessant and varied changes of 

 form which it exhibits (Gr. amoibos, changing). The Amceba is a 

 little microscopical creature which may commonly be detected in 

 stagnant water, especially where there is decaying vegetable matter. 

 When examined under the microscope, all that would probably be 

 seen at first would be a shapeless or irregularly spherical mass of 

 gelatinous, jelly-like sarcode, containing scattered granules. Soon- 

 the creature might be observed to push out a finger-shaped pro- 

 longation of its own substance ; and it would soon be found that 

 similar processes or pseudopodia could be pushed out at will from 

 almost any point of the body and again retracted within it without 

 leaving any trace behind. As a result of this, the form of the 

 animal is constantly changing, and hence its common name of 

 Proteus-animalcule (fig. 3 and fig. 6 B). By means of these tem- 

 porary processes of sarcode, the Amoeba both moves and obtains 

 food. Locomotion is effected in a kind of creeping manner, the 

 animal pushing out the pseudopodia in one direction and then 



