OEGANISATION OF SPONGES. 



385 



The Porifera, or Sponges, were formerly supposed to belong 

 fco the vegetable kingdom, but their animal nature is now fully 

 ascertained, for modern researches have proved that the soft 

 glairy substance with which their skeleton is invested during 

 life consists of " sarcode," similar to that which forms the soft 

 parts of the Foraminifera and PolycystiDa. It is by this- 

 animated or organic gelatine, which can generally be pressed 

 out with the finger, and in some species is copious even to 

 nauseousness, that the solid parts of the sponge are deposited, 

 and from it the whole growth of the mass proceeds. The 

 framework or skeleton of the Porifera is usually composed of 

 horny fibres of unequal thickness, which ramify and interlace 

 in every possible direction, anasto- 

 mosing with each other so as to 

 form innumerable continuous cells 

 and intricate canals, the walls of 

 which in the recent sponge are 

 crusted over with the gelatinous 

 living cortex. 



Generally this fibrous mass is in- 

 terwoven with numerous mineral 

 spicules of a wonderful elegance and variety of forms, for their 

 shapes are not only strictly determinate for each species of 

 sponge but each part of the sponge, it is believed, has spiculse 

 of a character peculiar to itself. Sometimes they are pointed 

 at both ends, sometimes at one only, or one or both ends may 

 be furnished with a head like that of a pin, or may carry three 

 or more diverging points, which sometimes curve back so as to 

 form hooks. Sometimes they are triradiate, sometimes stellar ; 

 in some cases smooth, in 

 others beset with smaller 

 spinous projections like the 

 lance of the saw-fish. In 

 many species they are 

 embedded in the horny 

 framework ; in others, as, 

 for instance, in Tethea 

 Cranium, or in Halichon- 

 dria, they project from its surface like a tiny forest of spears. 

 They are generally composed of silex or flint, but in the 



Single interspace or open cell, and 

 surrounding finer meshworx of 

 the skeleton of a sponge. 



Needle-like and starred spicula of a Tethea. 

 (Highly magnified.) 



