MATERIALS FOU A MONOGRAPH OP THE ASCONS. 487 



The closed pore-cells are a very obvious feature of contracted 

 Ascous, and with increased contraction of the sponge the pore- 

 cells go through a remarkable series of movements. Situated 

 at first in the dermal layer on a level with the inner ends of 

 the collar-cells, they push their way inwards between the latter. 

 If the contraction goes still further, as it does commonly in 

 the species without gastral rays, the collar-cells are forced one 

 over the other, and form a layer two or three cells deep ; 

 during these changes, or before, the porocytes migrate com- 

 pletely inwards, and form a granular epithelium covering over 

 the collar-cells, and lining the gastral cavity. Finally in the 

 extreme state of contraction the tubes become solid, filled 

 completely by the collar-cells, in the centre of which the 

 pore-cells are to be found forming, as it were, a granular axis 

 to the tube. As the sponge expands again all these move- 

 ments are gone through in reversed order. 



The contracted porocytes have attracted considerable atten- 

 tion, as might have been expected, from those who have 

 studied calcareous sponges, and unfortunately they have also 

 received very different interpretations. They seem to have been 

 first clearly described by Metschnikoff (1879) in Clathrina 

 clathrus and primordialis in specimens which, as is 

 evident from the figures and descriptions given of the 

 dermal epithelium, were in a state of complete contraction. 

 Metschnikoff described with his usual accuracy the appear- 

 ance of these cells, chiefly as seen in the living condition ; 

 their large size and compact form, their densely granular pro- 

 toplasm, and the colour of their granules, which were yellow 

 in clathrus, dark brown by transmitted light in primor- 

 dialis, like the perfectly similar granules in the "ectoderm" 

 (1. c. pp. 360, 361, pi. xxii, figs. 1, 2, 5, and S). Nevertheless 

 Metschnikoff" regarded them as mesoderm cells, which is the 

 more remarkable since he represented them quite correctly 

 lying between, not under, the collar-cells (1. c. fig. 2). He 

 believed them to give rise to the triradiate spicules, or rather 

 he confused the cell masses which form the spicules with these 

 cells, which, as we shall see, was an error. 



