26 ZOOLOGY 



stage can be found between a cell which has a large epithelial 

 portion wedged in between neighbouring epithelial cells, and a 

 cell in which this portion is reduced to a mere button, ' and 

 which consists mainly of the fibre. Such a cell is almost un- 

 distinguishable from a simple spindle-shaped muscular fibre. 

 Contraction in the sea-anemones is far more rapid than in the 

 sponges; and this is due to the fact that definite muscular 

 fibrils or myonemes are developed in the one case and not in 

 the other. 



We conclude, therefore, that epithelial cells are the funda- 

 mental type of cell, and that glandular and muscular cells are 

 derived and modified types. But epithelial cells can undergo 

 other modifications which are equally important and interest- 

 ing. An animal reacts i.e. alters its behaviour in conse- 

 quence of changes in its surroundings. These changes such 

 as changes of temperature, the impact of foreign bodies, and of 

 light and sound, &c. impress the animal through its outer 

 epithelium, and we find, therefore, that certain cells of this 

 epithelium are specially modified to receive and transmit these 

 impressions. Such cells are termed sensory cells (see Fig. 3). 

 In the simplest variety of sensory cells each possesses one or more 

 sense hairs projecting from its outer surface, which can be 

 made to vibrate by sound, or which when pressed on act as levers, 

 to transmit pressure into the interior of the cell. The visual cell, 

 which is affected by light and which forms the essential element 

 in all organs of sight, possesses a clear glassy rod instead of a 

 stiff hair. How the light irritates the visual cell is not known, 

 but it is known that another kind of cell is required before the 

 sensation of light can be received. This cell is a pigment- 

 cell, and its characteristic is that it secretes granules of an 

 intensely deeply-coloured substance termed pigment. Visual 

 cells and pigment cells are placed either side by side or end to 

 end. The suggestion has been made that the intensely rapid 

 vibrations which constitute the waves of light decompose the 

 pigment and produce some active chemical substance which 

 irritates the glassy rod, but this is mere conjecture. The cells 

 which are sensitive to odours have delicate hairs like cilia 

 (see p. 15) projecting from their surfaces, but we have as yet 

 no idea how the infinitesimal particles which constitute the 

 substance of a scent affect these hairs. 



