12 



NATURAL HISTORY. 



Fig. 10. — Diagram of more com- 

 plex sponge; letters, as in Fig. 9, 

 except e, cloaca; o, osculum. 

 Each of the chambers, «, cor- 

 responds to the whole of Fig. 9, 

 hut the scale is so small that the 

 collared cells can only be repre- 

 sented diagrammatically. 



Connected with the mesoderm and ectoderm is the formation of the 



skeleton. In some forms only one of these layers is concerned, in others 



the other, while in 

 still others both 

 layers may con- 

 tribute. In the 

 sponges which are 

 used for domestic 

 purposes only the 

 mesoderm is in- 

 volved. The cells 

 of this layer se- 

 crete horny fibres, 

 and it is these 

 fibres alone that 

 form the sponge 

 as sold in the 

 shops. The ecto- 

 dermal skeletal el- 

 ements are smaller and more regular in 



shape, and but rarely do they form a fine, 



solid network capable of retaining its form 



after the soft parts have been removed. 



In different sponges, or even in the same 



sponge, they may differ greatly in shape, 



but from the fact that usually they are 



more or less needle-like in shape they are 



called spicules. Our cut shows a few of 



the forms. 



According to the system most in vogue, 



sponges are divided into groups according 



to the nature of their skeleton. Those 



which have no skeleton are Myxospongise ; 



those with calcareous skeleton, Calcispon- 



giae ; with horny fibres, Fibrospongise ; 



while the last group have a silicious skel- 

 eton, and are consequently known as Si- 



licispongise. Only the last two of these fig.il- 



groups need detain us longer ; but one of 



these is interesting from the economic importance of some of its members 



while the individuals of the other group are noticeable for their beauty. 



Different forms of sponge-spicules, 

 greatly enlarged. 



