CLASS I PICA TION. 



129 



downwards, and is moored by amoeboid processes from the granular 



cells, which likewise obliterate the blastopore. The granular cells lose 



their granules, for the larva is not yet feeding ;"the now internal flagella 



disappear in the absence of the stimulating water : a mesoglcea with 



spicules begins to be formed between the inner and outer layer, pro- 



bably by migrants from the latter. But this disadvantageous state of 



affairs cannot last. Pores open through 



the walls, the entrance of water enables 



the inner cells to recover their flagella, 



and an exhalant aperture is ruptured at 



the upper pole. The young sponge is 



now in an A scon stage, from which, by 



the outgrowth (?) of the inner layer into 



radial chambers, it passes into the 



permanent Sycon form, grows into a 



cylinder, and becomes differentiated in 



detail (Fig. 58). 



{b~) In Oscarella (Halisarca) lohularis 

 (Fig. 59), a sponge without any skeleton, 

 the ovum segments equally into a 

 blastula, which is flagellate all over. 

 This free - swimming stage may be in- 

 vaginated from either pole to form a 

 hemispherical gastrula, which settles 

 mouth downwards. Pores, an osculum, 

 and the mesoglcea are formed as before, 

 and the inner layer becomes folded into 

 flagellate chambers. 



(c) Another type, seen for instance 

 in a horny sponge, Spongelia, results in 

 a flagellate larva, whose cavity is filled 

 up with what may be called gelatinous FIG. 59. Diagrammatic re- 

 connective tissue, from which mesoglcea presentation of development 

 and inner layer are subsequently dif- 

 ferentiated. Such a larva is called a 

 parenchyimda. 



As these are not all the types of 



lobularis. 



of Oscarella 

 After Heider. 



Bl. , Free-swimming blastula with 

 flagella; G., gastrula 

 down. 



settled 



development which occur among sponges, Xext figure shows folding of 

 the general fact is impressive, that in innerlayer(.;/.); /ic. , outer layer. 

 this lowest class of Metazoa there has L west 



j v i i . i 



been considerable plasticity in develop- 

 ment. 



s dl J*am; 



bers (R.C.}\ Mesoglcea (Mg.) , 



inha ia nt p 0re (/>.); exhalant 

 osculum (6>.). 



Classification.- 



Class I. CALCAREA. With skeleton of calcareous spicules : 

 Grade I. Homoccela. -- Continuous internal layer of collared 



flagellate cells, e.g. Ascetta, Leucosolenia. 

 Grade II. Heteroccela. Collared flagellate cells restricted to 



radial tubes or chambers, e.g. Sycon (Grant ia.} 

 Class II. HEXACTINELLIDA, or Triaxonia, with sexradiate siliceous 



