128 EUSPONGIA. 



The rigidity is subject to variations within eeri;ain limits. The skeletons of 

 all varieties are more or less elastic, and rebound when thrown against a hard 

 surface. The utility and commercial value of the varieties depend principally 

 on their elasticity and tenacity. E. o. mollissima and E. o. irregularis are more 

 elastic than the others. The least elastic form is E. o. chira. E. o. rotunda 

 is much less tenacious than the other varieties. All the forms, with the excep- 

 tion of E. 0. dura, are pretty soft and compressible. The last-named appears 

 hard and resisting, and is useless for practical purposes. 



In the concave fields between the low ridges which connect the conuli 

 a network of bands composed of spindle-shaped, probably muscular, cells 

 is observed ; these radiate from the conuli, ramify, and are connected 

 by numerous transverse bands. In the meshes of this primary network 

 secondary reticulations are sometimes observed. The meshes are occupied by 

 perforated membranes. Below these, large subdermal cavities, diiferently 

 developed in the different varieties, are observed. They may appear nearly 

 continuous, very extensive, and interrupted only by the stout columns of 

 tissue which support the conuli and the narrow bands connecting the 

 muscular trabeculse in the skin with the internal tissue ; or they may be 

 reduced to mere extensions or inflations of the inhalant canal-stems, as in 

 E. 0. spinosa. The largest subdermal cavities are observed in E. o. nitens. The 

 small grooves in the surface of the skeleton of certain forms, particularly 



E. 0. mollissima, are occupied in the living sponge by very large canals, which 

 belong to the system of subdermal cavities. Erom these subdermal cavities 

 large inhalant canal-stems extend down into the interior of the sponge. In 

 most of the forms these are large and conspicuous, on an average 0-5-1 millim. 

 wide and 2 millim. apart. They show numerous annular constrictions, which 

 divide them into series of more or less spherical inflations, so that the whole 

 canal attains an appearance somewhat similar to that of the caecum of a 

 rabbit. The small inhalant branch canals originate from these inflations, 

 and the entrances to them are often so abundant and close together that 

 these parts of the canal-walls appear sieve-like. The final ramifications of 

 these canals do not exceed O'Ol millim. in width ; they open direct into the 

 ciliated chambers, which are pear-sbaped, 0-03-0-04 millim. wide, and con- 

 stricted towards the efferent aperture, which leads into a small special 

 efferent canal, having the appearance of a neck belongiug to the chamber. 



F. E. Schulze is of opinion that there are several very small afferent pores 

 leading from the final branches of the inhalant canals into the ciliated chamber. 

 These pores are simple perforations in the convex aboral part of the ciliated 

 chamber. 



The necks of the ciliated chambers, the small special efferents, open into 

 larger canals, which join to form oscular tubes 2-5 millim. wide. Theso 



