78 SPONGES 



hatches in the condition in which the larva of Leucosolenia fixes 

 itself. 



In the Demospongiae it is not possible as ) r et to trace so complete 

 an evolutionary series as in the Calcarea, since the gaps in our know- 

 ledge are still very great. No larvae are known amongst Tetrac- 

 tinellids or Aciculina, 1 while amongst Clavulina only Cliona, and 

 amongst Dendroceratina only Aplysilla have been studied. On 

 the other hand, the life-history of some of the more primitive types, 

 such as Oscarella and Plakina, and of the Cornacuspongiae (Hali- 

 chondrina and Keratosa) have been the subject of careful investiga- 

 tions. As a convenient starting-point the development of Oscarella 

 may be selected. 



Total and regular segmentation leads in Oscarella to the forma- 

 tion of an egg-shaped blastula, with a relatively thin wall which is 

 composed of a single layer of columnar flagellated cells. Over the 

 broader anterior half of the embryo the cells are shorter, and 

 consequently the wall thinner than over the narrower posterior 

 half ; the spacious internal cavity is stated to contain no cells. In 

 this condition the larva is born into the world, and swims freely 

 for from twenty-four hours to three days. The anterior half or 

 two-thirds of the larva is yellowish in colour, the posterior portion 

 carmine red, Avith a dash of brown. During the larval life the 

 differentiation of the germ layers takes place. The thin-walled 

 anterior half, the future gastral layer, remains unmodified. The 

 thick-walled posterior half, on the other hand, destined to become 

 the dermal layer of the sponge, is the seat of considerable change. 

 The cells in this region become more granular and of compact 

 cubical form, and a certain number of them retract their flagella, 

 become amoeboid, and immigrate into the internal cavity (Fig. 

 59, 4). The majority of the dermal cells, however, remain at the 

 surface, and retain their flagella, a point in which Oscarella differs 

 markedly from Clathrina, and which is correlated with the fact 

 that in the former the dermal epithelium is ciliated throughout life. 

 In consequence the internal cavity is very far from being filled up, 

 and the larva, though now comparable to an amphiblastula, remains 

 uniformly ciliated all over the surface. Observations upon the 

 archaeocytes remain to be made. The larva thus constituted fixes 

 by the anterior pole, and the gastral cells become invaginated and 

 surrounded by the dermal cells. 



In Plakina the segmentation is total and regular, and the larva 

 emerges as an egg-shaped blastula of a rose-red colour, rather deeper at 

 the narrower posterior end. The body wall is made up of columnar 



1 Since Cliona is known to extrude ova, which segment and develop into larvae 

 outside the body, it is possible that the same mode of development explains the 

 apparent absence of larvae in other Clavulina and in Tetractiuellida, etc. 



