26 THE ELEMENTARY NERVOUS SYSTEM 



due not to changes in the activity of the choanocytes but 



to other causes. 



Some sponges, such as the fingered form Stylotella, 



appear, when out of water, to be more or less shrivelled 



or contracted and under other circumstances to be plump 



and well rounded-out. The 

 differences, which, for rea- 

 sons to be mentioned pres- 

 ently, are known not to be due 

 to the simple physical loss of 

 fluid, are apparently depen- 

 dent upon a general contractil- 

 ity of the whole flesh of the 

 sponge which, though slight, 

 may nevertheless enable the 

 sponge to change its form 

 somewhat. Aristotle in the 

 fourteenth chapter of his fifth 

 book on the history of animals 

 makes the interesting state- 

 ment that the sponge is sup- 

 posed to possess sensation 

 because it contracts if it per- 



ceives anv movement to tear 







it up and it does the same when the winds and waves are 

 so violent that they might loosen it from its attachment. 

 He further adds in his characteristic way that the natives 

 of Torona dispute this. 



The idea that the common flesh of the sponge is con- 

 tractile is not without modem support. Merejkowsky 

 (1878) stated that if the sponge Suberites is so placed that 

 it is partly out of water, it will curve the body until it is 

 under water as much as possible, and if the body is then 



Fio. 4. Diagram of the canal sys- 

 tem of a calcareous sponge (modified 

 Haeckel). The innumerable superfi- 

 cial pores receive water from the exte- 

 rior, as shown by the arrows on the 

 sides; the osculum at the apex dis- 

 charges water to the exterior. 



