1'ORIFERA. 



201 



CLASS III.— POKIFERA. 



The Porifera or Sponges (called Amorphozoa by De Blainville, by 

 reason of their diversity of forms), have a structure which is a union 

 of transparent pulpy matter (sarcode) with tough horny fibre. This 

 latter forms a branching net-work or skeleton throughout the entire 

 organism. In most Sponges the skeleton is further strengthened by cal- 

 careous or siliceous spicula. Reproduction takes place by gemmules, 

 and the young Sponge moves about for a time as a free organism ; but 

 after it attaches itself to some submarine rock, it shows no signs of 

 movement or sensibility. The only evidences of life are its growth and 

 the constant circulation of water — flowing in through the fine pores and 

 discharged through the larger tubes. Excepting the fresh-water Spon- 

 gilla, Sponges are natives of the sea. 



Pictet enumerates 47 genera and over 400 species of fossil Sponges, 

 which he groups in the three families of Spongidse, Clionidoe. and 

 Petrospongidse. The first includes the living forms with horny skeleton, 

 and the sole evidences of their existence in geological times are myriads 

 of minute siliceous spicula which largely pervade the mass of some 

 rocks. They are first found in the Silurian, and many layers 

 in the Cretaceous strata are almost entirely composed of them. The 

 Clionidse are also Sponges of horny structure, which pierce the shells of 

 Molluscs and the stems of Corals, and dwell parasitically in the cavities 

 which they have bored. These cavities and the silicified casts of the 

 perforations called " Clionites" quite riddle the valves of some shells 

 which lived in the Cretaceous age. The Petrospongidse had a reticulate 

 stony frame-work without Spicula, pierced with pores and oscula as in 

 the living Sponge. They have no representative at the present day, but 

 abounded in the seas of all geological periods from the Silurian to the 

 early Tertiary. 



No. 1164. Amorphospongia 



TMs undescribed specimen of a 

 large and remarkable fossil sponge, with 

 eleven beautifully radiating and recurv- 

 ed hollow branches, is from the Lower 

 Silurian, Franklin Co., Ky. 



Size, 11 x 11." Price, $3.25. 



