58 SPONGES 



archaeocytes correspond to the germ cells of other Metazoa, 

 but stand on a lower grade than those of any Enterozoa, 

 in so far as the germinal cells here are not idle cells, set apart 

 and biding their time to develop, but actually work for the whole 

 cell colony, performing elementary functions of digestion, distribu- 

 tion, and probably excretion, like leucocytes in other animals. In 

 sponges, to be brief, a leucocyte which has worked for the organism 

 may become a germ cell. In other animals leucocytes and germ 

 cells form two distinct classes of cells, though in Echinodenns at 

 least they appear to have a common origin. 



In accordance with these important facts the archaeocytes may 

 be considered from two points of view : first, as wandering cells, or 

 amoebocytes ; secondly, as reproductive cells, or tokocytes. These two 

 categories are not, however, to be regarded as two distinct classes 

 of cells, but simply as two different phases in the activity of one 

 and the same kind of cell. 



(a) Amoebocytes. The wandering cells of sponges are, as a rule, 

 easily distinguished from other cells of the parenchyma by their 

 lobose, rounded appearance, and the quantity of granules with which 

 their cytoplasm is usually packed, and which obscure the nucleus 

 in a general view of the cell. Very frequently more than one kind 

 of wandering cell can be distinguished, according to the nature 

 of the contained granulations, one kind having coarse, large 

 granules, the other fine granules, as in Clathrina contorta. Since 

 these granules are certainly to a great extent dependent upon the 

 state of metabolism of the sponge, these differences may correspond 

 only to variations in the functional activities of the same cell. In 

 other cases, however, differences of function appear to have led to 

 the establishment of well-marked and constant structural differences 

 between the cells, which may affect both nucleus and cytoplasm. 

 Thus in Sponriilla, Fiedler (1888) has described two kinds of 

 wandering cells which he has termed " Fresszellen " (phagocytes) 

 and " Xiihrzellen " (trophocytes) respectively; the former which 

 occur always near the free surfaces of the sponge body are 

 concerned more especially with the iugestion, and perhaps 

 digestion of food ; the latter, found in all parts, appear to 

 provide for its distribution. To these two classes must be added 

 a third, belonging really to the class of trophocytes but specially 

 charged, apparently, with the function of storing reserve material, 

 and hence conveniently termed thesocytes (Sollas). 



It is by no means beyond a doubt that the two classes of wandering 

 cells distinguished by Fiedler have exactly the function which he attri- 

 butes to them. The trophocytes frequently contain diatoms, and various 

 bodies apparently of the nature of food particles taken up by them ; 

 hence their function may perhaps be phagocytic as well as trophocytic. 

 Fiedler's phagocytes, on the other hand, may possibly possess an excretory 



