124 



PORIFERA SPONGES. 



and outer layer, probably by migrants from the latter. But this dis- 

 advantageous state of affairs cannot last. Pores open through the 

 walls, the entrance of water enables the inner cells to recover their 

 flagella, and an exhalent aperture is ruptured at the upper pole. The 

 young sponge is now in an Ascon stage, from which, by the out- 

 growth (?) of the inner layer into 

 radial chambers, it passes into the per- 

 manent Sycon form, grows into a cylin- 

 der, and becomes differentiated in 

 detail (Fig. 34). 



(b] In Oscarella (Halisarca] lobularis, 

 a sponge without any skeleton, the 

 ovum segments equally into a blastula, 

 which is flagellate all over. This free 

 swimming stage may be invaginated 

 from either pole to form a hemispherical 

 gastrula, which settles mouth down- 

 wards. Pores, an osculum, and the 

 mesoglcea are formed as before, and 

 the inner layer becomes folded into 

 flagellate chambers. It may be sug- 

 gested that this folding is due to the 

 higher nutrition, and consequent dis- 

 proportionate growth, of the inner 

 layer, for a rapidly growing sac within 

 one of less rapid growth must become 

 folded on itself (HEIDER). 



(c] Another type, seen for instance 

 in a horny sponge, Spongelia^ results 

 in a flagellate larva, whose cavity is 

 rilled up with what may be called gela- 

 tinous connective tissue, from which 

 mesogloea and endoderm are subse- 

 quently differentiated. Such a larva is 

 called a parenchymula. 



As these are not all the types of de- 

 velopment which occur among sponges, 

 the general fact is impressive that 

 in this lowest class of Metazoa there 

 has been considerable plasticity in 

 development. 



FIG. 35. Diagrammatic 

 representation of develop- 

 ment of Oscarella lobularis. 

 (After HEIDER.) 



Bl. Free swimming blastula with 

 flagella ; G. gastrula settled down. 



Next figure shows folding of 

 Endoderm (En). 



Lowest figure shows radial 

 chambers (A\C.)- Mesoglcea (Mg) ; 

 inhalent pore (/*.); exhalent oscu- 

 lum (0.). 



Classification. 

 A. Port/era Calcarea, with skeleton 



of calcareous spicules : 

 Order I. Homocoela. Endo- 

 derm wholly composed of collared flagellate cells, 

 e.g., Ascetta^ Leucosolenia. 



Order II. Heterocoela. Endoderm consists of collared flagel- 

 late cells in radial tubes or chambers, and of flat 

 epithelium elsewhere, e.g., Grant ia, Sycon. 



