PORIPERA. 



73 



nuclei, and presenting differentiations of various kinds-^ so-called 

 muscle cells, amoeboid cells, generative cells, scleroblasts {spicule- 

 forming cells). The mesodermal network is continuous both with 

 the ectoderm and with the endoderm ; indeed, these layers>may 

 fairly be regarded as superficial bounding expansions of the meso- 

 dermal mass. Skeletal structures, the main function of which is 



FIG. 61. Euspongia qfficinalis adriutica, with a number of 

 oscula, (after F. E. Schulze). 



FIG. 60. Amnella polypvides 

 (after O. Schmidt). 



FIG. 62. A branched Ascon-colony (after Haeckel). 



to support the sponge-body, are contained in the mesoderm. These 

 may be calcareous or silicious, in which case we get the so-called 

 sponge-spicules ; or they may consist of a horny material called 

 spongin (common bath sponge, Fig. 63); or finally, spongin fibres 

 and silicious spicules may co-exist. The generative cells are budded 

 off from the mesodermal network, and are eventually dehisced into 



