203 ANATOMY AND PHYSIOLOGY 



Silicea. For more minute information regarding its struc- 

 tural peculiarities, I must refer my readers to the following 

 description of the generic characters of Diplodemia. 



DiPLODEMiA, Bowerbank. 



Skeleton fibrous. Fibres keratose, hetro-spiculous ; com- 

 bined with a secondary skeleton of irregular network 

 of spicula ; rete unispiculate, rarely bispiculate. Ova- 

 ries membranous and spiculous. 



Type, Diplodemia vesicula, Bowerbank. 



The fibres in the skeleton of the only known species in 

 this genus are very remarkable. They are smooth and 

 cylindrical, having an axial line of, generally speaking, 

 single spicula united at their points, running throughout 

 the whole length of the fibre. But when it is of more than 

 ordinary diameter, there are frequently other spicula at 

 intervals imbedded in the fibre parallel to the axial series. 

 Throughout the whole length of the fibres, at short inter- 

 vals, there are similar spicula to the axial ones, imbedded 

 at right angles to the axis of the fibre, frequently project- 

 ing from the surface for half, or more than half their length. 

 Some of these projecting spicula originate small lateral 

 branches of the keratose skeleton, but by far the greater 

 portion of them are the connecting points of the keratose 

 fibres and the reticulo-spiculate secondary skeleton ; the 

 former being thus completely imbedded amidst the latter. 



The structure of the ovaria in this genus is also peculiar 

 to it. The wall is very thin, and appears to consist of a 

 single membrane profusely furnished with spicula which 

 cross each other in every direction, and occasionally appear 

 to assume a somewhat fasciculated arrangement. They 

 are not uniform in shape, some being regularly oval, while 

 others are more or less ovoid. 



But one species of this singular genus is known, D. 

 vesicula, Bowerbank, from deep water at Shetland. Fig. 



