728 AK ALGA, FORMING A PSEUDOMORPII OF A SILICIOUS SPONGE. 



ous spicules — that these structures are Pseudomorplis of the Dactj- 

 lochalina australis. I assume, that the Alga is a pai-asitic species 

 growing in the sponge, and extending throughout the whole body of 

 it. The sponge is thereby resorbed by the Alga. The soft parts and 

 horny fibi-es disappear, whilst the silicious spicules are left, and 

 appeal', on close examination, adhering to the outer side of the stem 

 and branches of the Alga. In this way this Alga forms a true 

 Pseudomorph of the sponge. 



I do not think it in any way comparable with other more simple 

 Alga-parasites found in sponges, which cause the formation of the 

 filaments in the Thicinidse, and others. 



EXPLANATION OF THE FIGURE. 

 (Plate XLVIII., Fig. 5.) 

 Section throiigh a portion of the Pseudomorph magnified. 



