544 GENERAL CHARACTERS OF PROTOZOA. 



a particular spot, which may be regarded as an anus. Their 

 reproduction usually takes place by a division of the substance 

 of the creature itself; sometimes, as in the Hydra, by a process 

 of gemmation, whilst in some instances a production of germs by 

 the division of the nucleus has been observed. 



1198. It is in these animals that we find the nearest approach 

 to the vegetable kingdom ; in fact, the lowest of the Protozoa 

 approach so closely to the most imperfectly organised plants that 

 it is at times difficult to say to which kingdom of nature a crea- 

 ture may belong. Many of the organisms included by Ehrenberg 

 amongst the Infusorial Animalcules, which constitute the most 

 important class of the Protozoa, have been since proved to be- 

 long to the vegetable kingdom ; and the researches of Mr. 

 Carter, of Bombay, go far to show that under certain circum- 

 stances the sarcode-like parenchyma of particular plants may 

 acquire an appearance and behaviour so like those of some 

 Protozoa, as almost to render it doubtful whether any boundary 

 can ever be established between the two kingdoms of organic 

 nature. 



1199. Almost all these creatures are inhabitants of water, 

 only a few being found as parasites in the viscera of other animals. 

 They exhibit both simple and compound forms, and in some of 

 the latter the union of the individuals is effected by exceedingly 

 curious means, and accompanied by remarkable phenomena of 

 community such as we meet with in no other group of animals. 

 Many of them have the power of secreting a shell or case, which 

 is either of a horny or calcareous nature ; the remains of the 

 latter occasionally constitute the greater part of large beds of 

 rock. 



1200. The Protozoa may be divided into three Classes : 



I. INFUSORIA, or Infusorial Animalcules, so called from their 

 having been first discovered in vegetable infusions ; they have 

 a firm integument, furnished with cilia by which they swim 

 through the water, and nearly all of them take their food through 

 a mouth, which communicates by a short oesophagus with the 

 cavity in their interior. 



II. RHIZOPODA, in which the body appears to consist of nearly 



