A GUIDE TO THE SPONGES 233 



rapid. The blastopore closes ; the flagellate cells develop collars 

 and become choanocytes; the osculum or excretory opening 

 perforates the free end; the side walls are pierced with pores; 

 traces of the skeletal spicules begin to show in scattered meso- 

 dermal cells as tiny needles of glass or carbonate of lime; and the 

 body assumes a somewhat cylindrical shape. From now on the 

 animal possesses all the elements of a true sponge, and growth 

 proceeds according to its nature and environment. 



PHYSIOLOGY. 



This subject, in its application to sponges, is very imperfectly 

 known. 



The following facts, however, can be definitely stated : 



The adult sponge is attached and is incapable of locomotion. 

 Its only outward movements seem to be a slow dilatation and 

 contraction of the pores and the osculum. 



The choanocytes, however, are very active. The flagella 

 are in constant vibration, and the collars are continually ex- 

 panding and contracting. These cells are the chief organs of 

 nutrition and respiration. The motion of the flagellum creates 

 a whirlpool, by means of which the sea-water and the organisms 

 it contains are sucked down within the collar. The cell then seizes 

 upon, and absorbs the digestible organisms, while the constantly 

 renewed sea-water, being brought into closer relation with the 

 absorbing tissues, causes the necessary oxygenation to take 

 place. 



Excretory products are, without doubt, cast out by these 

 cells and together with the indigestible organisms are borne out 

 through the osculum by the main current of sea-water. 



It is also said that during the winter many choanocytes dis- 

 appear, to be restored in the spring-time. Thus a kind of hiber- 

 nation seems to occur. 



The growth of sponges is slow, five or six years being neces- 

 sary to bring them to their full size. This, however, is very 

 variable. 



There is no muscular or nervous system. Instead, there is 

 what has been called a ' 'vague general sensibility" of the whole 



