30 SPONGES 



proceeding pari passu with a thickening of the body wall, and 

 resulting in a considerable diminution in the size of the sponge as 

 a whole. When the contraction is carried to its extreme, the 

 gastral cavity disappears altogether and the interior of the sponge 

 is filled by a solid mass of cells. 



During the contraction of the sponge, the arrangement of its cell 

 elements undergoes great changes, which are very important for interpret- 

 ing the early stages of the embryonic development. The collar cells 

 become first laterally compressed and very columnar (Fig. 42, E\ and 

 finally are forced over one another into several layers (Fig. 42, F, chc}. 

 During these changes the collar shortens, and is finally completely 

 retracted. The spicules are also forced one over the other to form several 

 layers. The porocytes, which at first were lodged in the body wall below 

 (external to) the collar cells, pass between the latter (Fig. 42, E), and finally 

 take up a position over (internal to) the collar cells (Fig. 42, F), forming 

 an epithelium lining the now greatly reduced gastral cavity. When the 

 contraction reaches the stage in which the gastral cavity is completely 

 obliterated, the collar cells and porocytes fill the gastral cavity as a 

 compact mass of cells, the porocytes being aggregated towards the centre, 

 or rather the axis, of the sponge. Lastly, the cells of the dermal 

 epithelium, the active agents in bringing about the contraction, them- 

 selves undergo a remarkable change of form. As the cell contracts, the 

 nucleus and the central protoplasm travel inwards towards the mesogloea, 

 while the peripheral portion of the cell, on the contrary, becomes raised 

 up. In this way the cells lose the flattened plate-like form which they 

 have in the expanded condition (Fig. 42, D) and assume each a shape 

 rather like a mushroom, the nucleus being lodged at the base of the stalk 

 (Fig. 42, F). 



When a contracted Ascon expands again, all the above changes of 

 structure are repeated in reverse order. The gastral cavity appears in 

 the midst of the porocytes which at first form an epithelium lining it, 

 and as the expansion continues, the porocytes become separated and 

 isolated from one another, and then travel outwards to take up their 

 position in the wall and to form pores. 



Contractility to a greater or less degree is found in all sponges, 

 but, so far as is known, it is only in the more primitive species of 

 the genus Clathrina that it is carried to the extreme degree of 

 obliterating the gastral cavity, and so producing a condition com- 

 parable to the pupal stage in the development (cf. Figs. 58, 2, and 

 63, B}, In those species of the genus which have spicules project- 

 ing into the gastral cavity, contraction is never carried so far, while 

 in the majority of sponges the phenomena of contraction are only 

 manifested in the temporary closure of the pores and oscula, both 

 of which structures, but especially the former, readily disappear 

 and appear again. The condition, however, in which an Olynthus 

 or any other sponge appears without osculum and pores is always a 

 temporary one. 



