52 INVEETEBEATA CHAP, m 



larva, instead of creeping about seeking fresh food, holds on with its 

 protective cells and lets the current waft fresh food into its reach. 



So far our reasoning appears safe. But the porocytes baffle 

 explanation ; we cannot picture to ourselves a process by which cells 

 converted themselves into drain-pipes, when we remember that 

 every step in the process must have been functional and must have 

 had a survival value. We can only imagine that the hollowing out 

 of a cell is perhaps a shortened reminiscence of the process by which 

 gaps in the attached rim, which must have existed to allow the 

 ingress of water, became surrounded by protoplasm. The need for 

 extending the surface of absorption, once fixation were accomplished, 

 would account for the extension of the area of flagellated cells by 

 their invagination, so that collectively they took on the form of a 

 cylinder ; but the formation of the osculum is utterly obscure. 



In some few cases we can compare ancestral history as recorded 

 by fossils, with ancestral history deduced from embryology ; we can 

 then see, as compared with the record deduced from fossils, what an 

 abbreviated sketch is constituted by the embryological record. In 

 the present case it is true we have no fossils to guide us, but the 

 abbreviation of ancestral history, as reflected in larval history, must 

 be intense. The later stages of the history of the race, the gradual 

 complication of the chamber system, is mirrored in the post- larval 

 development : the first fixed Gfrantia is at first an Ascon, and only 

 gradually takes on the Sycon characters as it grows in size. 



The history of all sponges just after fixation would be a most 

 interesting field for research, and would throw much light on their 

 mutual affinities. 



LITERATURE REFERRED TO 



Dendy, A. On the Pseudo-gastrula Stage in the development of Calcareous Sponges. 

 Proc. Roy. Soe. Victoria (Australia), 1898. 



Evans, R. The Structure and Metamorphoses of the Larva of Spongilla lacustris. 

 Quart. Journ. Mic. Sci., vol. 42, 1899. 



Evans, R. A description of Ephydatia blembingia, with an account of the formation 

 and structure of the gemmule. Quart. Journ. Micr. Sci., vol. 44, 1900. 



Maas, 0. Zur Metamorphose der Esperia lorenzi. Mitt, aus der Zool. Station zu 

 Neapel, vol. 10, 1892. - 



Maas, 0. Die Keimblatter der Spongien und die Metamorphose von Oscarella 

 (Halisarca). Zeit. fiir wiss. Zool., vol. 63, 1898. 



Maas, 0. Die Weiterentwicklung der Syconen nach der Metamorphose. Zeit. fur 

 wiss. Zool., vol. 67, 1900. 



Maas, O. tiber die Einwirkung carbonatfreier und kalkfreier Salzlb'sumgen auf 

 erwachsene Kalkschwamme und auf Entwicklungsstadien derselben. Arch. f. Ent- 

 wick., vol. 27, 1906. 



Maas, 0. Zur Entwicklung der Tetractinelliden. Verh. der Deutschen Zool. 

 Gesell., 1909. 



Marshall, W. The reproduction of Spongilla lacustris. Sitzungsber. Naturforsch. 

 Gesell. Leipzig, 1884. 



Minchin, E. Note on the Larva and the Post-larval Development of Lcucosolenia 

 variabilis, with Remarks on the Development of other Asconidae. Proc. Roy. Soc., vol. 

 60, 1896. 



Minchin, E. The Porifera. Lankester's Treatise on Zoology, vol. iii., 1900. 



Schulze, F. E. Untersuchungen iiber den Bau und die Entwicklung der Spongien 

 I. Mitt. Uber den Bau und die Entwicklung von Sycandra raplianus, Zeit. fiir 

 wiss. Zool., vol. 25, Suppl. 1875. 



