188 ANATOMY AND PHYSIOLOGY 



MICROCIONA, Bowerbanh. 



Skeleton. A common basal membrane, whence spring at 

 or about right angles to its plane numerous separate 

 columns of spicula intermixed with keratode, furnished 

 externally with spicula which radiate from the columns 

 at various angles towards the dermal surface of the 

 sponge. 



Type, Microciona atrasan guinea, Bowerbank. 



The skeleton of the type of this genus, M. atrasawf/uinea, 

 is different from that of any other genus of sponges that I 

 have hitherto seen. It consists of numerous, nearly equi- 

 distant, short, straight, separate columns of spicula and 

 keratode from all parts of the sides of which spring stout, 

 long, curved, fusiformi-attenuato-subspinulate spicula, the 

 convex side of each spiculum being outward, and each 

 column terminates with five or six of these spicula disposed 

 in the same manner and at the same angle to the axial line 

 of the column, that is from about twenty to forty-five 

 degrees. The proportions of the skeleton-columns vary in 

 different species. In M. atrasanguinea they are short, 

 stout, and exceedingly well defined. In M. ambit/ua they 

 are short and indistinctly produced, and in M. carnosa they 

 are long, slender, flexuous, and frequently branched ; but 

 however they may vary in their proportions in different 

 species, their normal character, both as regards structure 

 and position in the sponge, is always preserved. Fig. 368, 

 Plate XXXIII, represents a single column of the skeleton 

 of Microciona atrasanyuinea, Bowerbank, showing its struc- 

 ture and the proportions and positions of the external 

 defensive spicula, X 175 linear. Fig. 369, Plate XXXIV, 

 represents a section at right angles to the surface of the 

 sponge exhibiting the columns of the skeleton in situ ; a, the 

 plane of the dermal membrane with groups of tension 

 spicula. 



