ANIMALCULES 



491 



I. Vertebrata 

 II. Nemertea 



III. MoUusca 



IV. Arthropoda 

 V. Annelida 



VI. Gephyrea 

 VII. Rotifera 

 Invertebrata ■{ VIII. Molluscoida 

 IX. Platyhelmia 

 X. Nemathelmia 

 XI. Echinodermata 

 XII. Coelenterata 

 XIII. Porifera 

 .XIV. Protozoa 



Triploblastica' 



■ Metazoa. 



}■ Diploblastica ^ 



Protozoa, and consequently great interest attaches to the study 

 of those members of the latter group which are aggregates of 

 cells, since some of these may, in some respects, be like the 

 transitional forms. 



Returning to the consideration of Amoeba, we find that this 

 differs from a colourless corpuscle in possessing a liquid -filled 

 space within the protoplasm which is constantly changing its size 

 in a regular manner, and is therefore termed a pulsating vacuole. 

 If carefully watched in a living specimen placed under the micro- 

 scope, this will be found to slowly expand to a certain size and 

 then to suddenly contract so as to be entirely lost to view, soon 

 reappearing, however, at the same spot. It appears to communi- 

 cate with the exterior, and probably has to do with getting rid 

 of waste products {excretion) and perhaps also with the introduction 

 of dissolved oxygen into the body for the purpose of respiration. 



Amoeba is sensitive to external influences, being affected, for 

 example, by changes of temperature, but it also appears to be 

 able to execute movements, apart from the direct action of such 

 influences. Otherwise expressed, the minute fragment of proto- 

 plasm which constitutes the body of this animal, performs those 

 functions which, in higher forms, are relegated to a nervous system, 

 and its connected sense organs. 



Careful observation has shown that Amoeba multiplies itself by 

 a process of splitting ox fission, as observed in some animals higher 

 in the scale (pp. 476). The nucleus elongates, and becomes 

 divided into two parts, and at the same time the rest of the body 

 is halved, the result being that the parent animal disappears 

 altogether as an individual, being replaced by two new animals 

 which subsequently increase in size. 



