210 SCIENCE PROGRESS. 



of the larva as being thrown off, and the whole sponge as 

 arising from its inner mass, but this statement, now known 

 to be erroneous, did not serve to put things in any clearer 

 light. With regard to other facts in the metamorphosis of 

 the larva of Silicispongiae the statements of investigators 

 were very contradictory. Some authors described the 

 fixation as taking place by the non-ciliated pole, which was 

 regarded as the blastopore, and the whole larva was then 

 compared to the amphigastrida or invaginated stage of 

 Sycons, which had been clearly shown to fix itself by the 

 blastopore or orifice of invagination. But, according to 

 other authors, again the fixation of the larva took place by 

 the ciliated pole, and with regard to other details of the 

 metamorphosis the statements were equally at variance. 

 Moreover, certain of the lower forms such as Oscarella, 

 Halisarca and Plakina were shown to have a type of 

 development very different from that prevailing in Silici- 

 spongiae, and more resembling the process known to occur 

 in Ascons. 



It was therefore impossible, in the face of such 

 differences in the mode of development, to reduce the 

 embryology of sponges to any uniform scheme or funda- 

 mental type. The most that could be said with certainty 

 was that all sponges had a free swimming larva with the 

 surface formed either completely or partially of a layer of 

 flagellated cells, and that the larva soon became fixed and 

 developed into a young sponge. This larva more or less 

 resembled the planula larva of Ccelenterates, in which the 

 outer ciliated covering becomes the adult ectoderm, and in 

 a similar manner the ciliated cells of sponge larvae were said 

 in nearly every case to become the adult ectoderm. But in 

 one type, and that too the best known, namely, the amphi- 

 blastula, it was shown beyond all doubt that the ciliated 

 cells became the endoderm ; that is to say, they developed 

 into the layer of collared cells, which most zoologists were 

 agreed in comparing with the endoderm proper of Ccelen- 

 terata. This fact separated the amphiblastula larva from 

 all other known sponge or Metazoan larvae, and made it, 

 in particular, impossible to compare the amphiblastula with 



