194 AETHUE DENDY. • 



of the radial chambers^ but they are always arranged entirely 

 witliout regard to these. 



lu Grantia labyrinthica (9) we find minute surface oxea 

 in the gastral as well as in the dermal cortex. This is a very 

 unusual occurrence, but is paralleled in Ute Spenceri (4). 



In Grantia Vosmaeri (fig. 10), where we have a marked 

 shortening of the radial chambers and a corresponding elonga- 

 tion of the exhalant canals, together with a strong develop- 

 ment of the mesoderm surrounding them, we find the skeleton 

 in the inner portion of the sponge wall losing its regular 

 radially symmetrical character, and becoming difi'use and 

 scattered. This is an illustration of a general law, that the 

 skeleton of the chambers varies with the chambers themselves, 

 which will be found to hold good throughout the group, with 

 few exceptions. As the chambers lose their radial symmetry 

 and regularity of form and arrangement, so the skeleton loses 

 its radially symmetrical articulate character, and becomes 

 difi'use and scattered. This will be seen to follow as a natural 

 consequence, if we remember that the position of the spicules 

 is to a large extent determined by the position of the layer of 

 mesoderm (in the chamber wall) in which they are developed. 

 Hence it is that irregularity in the skeleton generally accom- 

 panies thickening of the mesoderm, for when this is effected 

 the spicules are free to take up a variety of positions which, 

 while embedded in a very thin layer of mesoderm, they are 

 unable to assume. 



Grantiopsis (fig. 11). 

 In Grantiopsis cylindrica (4) we meet with a slight 

 modification of the Grantia type of skeleton, as shown in 

 fig. 11. The dermal cortex is enormously developed, but its 

 skeleton still consists of triradiate and oxeote spicules arranged 

 as in Grantia. In the thin gastral cortex quadriradiates are 

 present as usual, but of somewhat peculiar shape, with small 

 facial and enormous apical rays. The subgastral sagittal 

 triradiates of the ordinary Sycon and Grantia types are here 

 replaced by subgastral sagittal quadriradiates. The position of 



