78 SPONGES 



hatches in the condition in which the larva of Leucosolenia fixes 

 itself. 



In the Demospongiae it is not possible as yet 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,^ while amongst Clavulina only Cliona, and 

 amongst Dendroceratina only AphjsiUa have been studied. On 

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

 such as OscdreUn and Phihina, and of the Cornacuspongiae (Hali- 

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

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

 may be selected. 



Total and regular segmentation leads in O.^carcUa to the forma- 

 tion of an egg-shaped blastula, with a relative!}' thin wall which is 

 comj)Osed of a single layer of columnar flngellated 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 ))ortion 

 carmine red, with 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 Hagella, 

 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 Oscnrella differs 

 markedly from Clafhriiin, and which is correlateil with the fact 

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

 In consequence the internal cavity is verj' far from l)eing 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 

 b}' the anterior pole, and the gastral cells become invaginatcd and 

 surroumled by the ilermal cells. 



In J'lukiiKi the segmentation is total and re;.jular, and the larva 

 emer^'es as an e<:;f;-sliaped bhistula of a rose-red colour, rather deeper at 

 tlie narrower posterior onil. The body wall is made up of columnar 



' Since Cliona is known to extrmle ova, wliich segment ami ilevelop into larvae 

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

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



