37 



— ■ in decalcified preparations — the sponge turned out to cover the branches and spaces between 

 the branches of the alga as a crust, closely fitting to its surface, or it was growing between 

 débris, consequently by and by enveloping the latter. In this connection it is perhaps worth 

 while to draw attention to Lendenfeld's statement that V. jlorida "bohrt sich in al te Korallen- 

 skelette". I believe, therefore, that our sponge does not perforate its substratum, probably 

 hardly dissolves the linie, but of course may enter preexisting holes. As far as I can see 

 Spirastrella purpurea begins its post-larval life by incrusting. 



Such incrusting stages are known from trop. tegcns. I cannot yet decide whether the 

 specimens I included here perhaps grow out afterwards to forms with digitate processes or to 

 massive forms. Provisionally I unite the incrusting Spirastrellae in this group. At any rate it 

 seems pretty sure that the crust, originally very thin, may become thicker, as we see in a S. 

 dccuDibcns" and in some Siboga specimens. There are in this collection several specimens which 

 correspond to Ridley's Spirastrella decumbens. The original type (B.M. 27) is said to form crusts 

 of 0.5 — 10 mm. thickness. "General surface level (except where affected by the inequalities of 

 the substance to which it is attached), glabrous". Later authors l ) usually describe S. decumbens 

 as incrusting. The specimen describecl by Ridley & Dendy (B.M. 38) seems to be more than 

 a mere crust; it is considered older, the original "as only a young specimen". The specimens 

 from the Siboga, mentioned above form crusts of about 1 mm. thickness, which may increase 

 in places where they fill so to say holes or deepenings of the substratum. Specimen 1031 

 is incrusting a coralline alga [Goniolithonï), which on its turn seems to cover an old piece 

 of coral. Specimens 1455 a-d 3 ) likewise grow over and between a coralline alga {Arcliacolitlio- 

 tkamnionï)\ they cover it for a great part (PI. IV, fig. 6). Numerous other animals and plants 

 are to be found in addition to Spirastrclla ; some are attached to it, others perforate and 

 destroy it, again others freely live in holes either of the substratum or of the sponge. Thus we 

 see e. g. Molluscs, Bryozoa, Corals, Annelids, Tunicates etc. fixed to the Archaeolithothamnion ; 

 we find the latter perforated and partly almost entirely destroyed by Thoosa and boring Algae 

 or Fungi; we see finally Errantia and Ophiurids more or less hidden in the holes. Of course 

 there exists a hard struggle for life between all these organisms, which is very interesting to 

 study. By the destruction of perforating organisms and other agentia numerous débris are formeel, 

 which are used by other animals as material for solidating their clwellings; an amount of débris 

 remains dispersed on the general surface, till our Spirastrella gets hold of them, grows over 

 them and envelopes them. Where our sponge finds a half destroyed portion of calcareous 

 matter, a pit, a hole etc. it creeps into it and fills it, but, as stateel before, never perforates 

 itself. Hence we frequently find calcareous and other débris in the parenchyma of the sponge 

 or stuck to its surface. Considerable portions of the sponge surface are, however, free and the 

 crust shows there the characteristic smooth, glabrous, shining or glossy appearance (PI. IV, 

 fig. 6). The external appearance of 1945 (PI. III, fig. 2) very much resembles that of 1455 a-d; 

 it incrusts a piece of coral, covered with a coralline alga, forming a block of about 9 by 15 cm. 



1) See for literature p. 12 — 13. 



2) The four specimens are very likely fragments of one and the same specimen. 



