[6] 



surface of the body. In these tiny Protozoa, those 

 organisms which consist of one single cell only, the 

 Amoebae, as well as those consisting of several cells in a 

 state of union, the Synamcebae, are able to perform all 

 the functions of animal life cohesion, sensation, motion, 

 digestion, and reproduction ; but, as the organism be- 

 comes more and more complex, these different functions 

 are shared among several groups of cells. This differ- 

 entiation proceeds steadily stage by stage, until at last 

 different senses are located in different parts of the body, 

 and we find animals possessing eyes, ears, noses, and 

 mouths, one organ performing the function of sight, 

 another that of hearing, and so on. All these organs of 

 sense are but parts of the general nervous organisation 

 of the body, which is apparently absent in the Protista, 

 but existing potentially in the protoplasmic substance, 

 as it also does in every other substance in the uni- 

 verse. 



The ciliated multiple cell-organism, in course of time, 

 becomes transformed into a hollow body, having a wall 

 composed of a single layer of cells, and this again, by 

 invagination, or folding of itself within itself, forms a 

 double-walled cavity, or Gastrula, having an external 

 opening like a mouth. These little animals, the Gas- 

 trceada, having an inner layer of cells (the endoderm), 

 which carries on the nutritive and assimilative functions 

 of the organism, and an outer layer (the ectoderm), 

 which forms the general motor and sense-organ of the 

 body, are the first animal organisms to possess a real 

 sense-organ separate and distinct from other parts of the 

 body. From this epidermal organ of sense are deve- 

 loped, as higher forms of animal life make their appear- 

 ance, the nerve-cells and sense-cells which form the 

 whole nervous system. 



In the fresh-water polyp, or Hydra, which is wanting 

 in distinct organs of sense and nervous system, we find 

 a remarkable sensitiveness to touch, warmth, and light, 

 individual ectodermic neuro-muscular cells performing 

 these functions, but a far greater sensibility being exhi- 

 bited in the circle of fine prehensible tentacles surround- 

 ing the mouth than elsewhere. Here we have a marked 

 attempt at localisation of sense-organs, and a manifesta- 



