in PORIFEKA 51 



which we shall become familiar when we study the next group, 

 Coelenterata. 



Maas (1906) lias reared the larvae of Calcareous Sponges in 

 water artificially deprived of all carbonate of lime. The result was 

 that no calcareous spicules were formed, and when the larva fixed 

 the flagellated cells formed a solid mass and developed no lumen. 

 Hence Alaas concludes that the formation of spicules acts as a 

 stimulus which determines the invagination of these cells to form a 

 hollow cylinder. This may be true for Grantia, and other Calcareous 

 Sponges, but it is obviously untrue for Oscarella which has no 

 spicules. 



ANCESTRAL HISTORY 



In the introductory chapter it was pointed out that there is 

 strong evidence that larval forms are, broadly speaking, reminiscent 

 of ancestral conditions of the stock or phylum. When we find in 

 the ontogeny of all sponges the blastula form cropping up, and 

 further find that, in those with the longest larval history this stage 

 is larval, becoming embryonic only in cases of a short free life, we 

 feel justified in assuming that it represents in a rough sort of 

 way the common ancestor of all Porifera. 



Such an ancestor a hollow vesicle of flagellated cells were it 

 now living would be termed a colonial Protozoon. In Volvox we 

 have an organism which, if it did not possess chlorophyll and live 

 like a plant, would correspond fairly closely to our idea of what this 

 ancestral sponge must have looked like. Now the great interest 

 attaching to the blastula is that it appears as a larva in the life- 

 histories of at least two other primitive groups of Metazoa, and that 

 as a more or less modified embryo it can be detected in the develop- 

 ment of all the Metazoau groups. Hence the case for regarding 

 it as representing the ancestral stock of Metazoa is greatly 

 strengthened. 



But such a stock when it existed must have been of world-wide 

 distribution, swarming in all the seas and waters of the globe. Such 

 a world- wide stock would become adapted to different " stations " 

 and just as at the present day we have bottom-feeding as well as 

 mid-water fish, so we may imagine that bottom-feeding blastulae were 

 developed. These, instead of devouring the floating and swimming 

 organisms like the rest, turned their attention to the microscopic 

 forms lying on the bottom. Under these circumstances only the 

 cells on the lower half of the blastula would be effective feeders, and 

 the more flattened this part became the more effective would be 

 their work. The other cells would become merely protective and 

 would tend to lose their flagella, and so the spherical blastula would 

 be modified into a cap-like form. Fixation would be the next step, 

 and so far as we can tell from a consideration of the life-histories 

 of fixed animals belonging to other phyla, fixation is an adaptation 

 to withstand and at the same time take advantage of currents. The 



