! PROTOZOA 13 



shape (amoeboid). At other points processes already formed are 

 withdrawn. The mass of the plasm flows towards the newly formed 

 processes, and locomotion thus appears as an irregular streaming. 

 In this way small foreign bodies are taken into the Amoeba body ; 

 if they can be assimilated they are digested, if not they are left 

 behind as the Amoeba moves forward. The Amoeba grows as its 

 nourishment increases. It multiplies by the nucleus becoming con- 

 stricted in the shape of a dumb-bell and finally dividing into two nuclei. 

 After complete division of the nucleus the plasm of the body also 

 becomes constricted and falls into two parts, each with its nucleus. 

 In this way each daughter Amoeba is, except in the matter of size, like 

 the mother. Reproduction by fission. 



I. Protoplasm. 



The protoplasm of many Protozoa (of certain Monera, of the 

 Pihizopoda, a few Amoeba, and most Flagellata) is tolerably homogeneous, 

 i.e. uniformly granular. In most cases, however, there is a differentia- 

 tion into an outer and an inner layer ; the former firmer, hyaline or 

 more often finely granular, and generally more contractile (cortical 

 layer, ectoplasm, eetosare) ; the latter more liquid and granular 

 (medulla, endoplasm, endosare). In some Heliozoa the endosarc is 

 the more homogeneous, the eetosare granular. There is generally no 

 sharply defined barrier between the two layers. In the Radiolaria 

 the protoplasm is divided by a membrane (capsular membrane) into 

 two parts, the extra- and intra-capsular protoplasm, which, how- 

 ever, communicate with one another by means of various perforations 

 of the membrane, and thus do not correspond with the ecto- and 

 endo-plasm above mentioned. In a similar way, in the RUzopoda with 

 calcareous shells, some of the protoplasm surrounds the shell. 



The capsular membrane possesses either numerous fine pores (Spumellaria, 

 Acantharia), or one single round aperture (osculum), with a porous cover or 

 operculum (Nasellaria), or, in addition to two or more apertures, one principal 

 aperture closed by a radially striated cover produced externally in the shape of a 

 tube (Phoeodaria}. 



Both ectoplasm and endoplasm are distinguished, in the case of 

 most Protozoa, by special structural modifications and differentiations. 

 The ectoplasm supplies the adaptations for locomotion and alimenta- 

 tion pseudopodia, cilia, flagella, suctorial tentacles which also serve 

 as feelers, oral and anal apertures. It frequently forms on its surface 

 a cell integument (cuticle) which may form the substratum of a 

 great variety of shell structures. The ectoplasm generally also supplies 

 the material for the various skeletons met with in many forms. The 

 contractile vacuoles and the stinging capsules (trichocysts), where such 

 are found, almost always lie in it. In some cases it gives rise to 

 special contractile portions (Infusoria, Vorticella). 



