ISODIOTYA. 117 



its length remaining attached to the body of the 

 sponge. In both specimens the caudal portion of the 

 sponge maintains the same diameter for the whole of 

 its length, so that the specimens closely resemble each 

 other, each having the form of a rudely-shaped child's 

 battledore. 



" This remarkable form is not without a precedent, as 

 it occurs in Polymastia radiosa represented in Plate 

 XI, figs. 14 and 15, vol. iii, ' Mon. Brit. Spongiadce ;' 

 and, what is very remarkable, there is about the same 

 amount of individual variation in the two specimens of 

 each species of sponge, so that if the figures represent- 

 ing P. radiosa were enlarged to the size of the two 

 specimens of Isodictya ferula, they would form very 

 good representations of the shapes of those of I. ferula, 

 so as to be very apt to produce a belief in the mind of 

 a hasty observer that the latter-named sponges were 

 enlarged specimens of P. radiosa, or vice versa ; but 

 accurate examinations of the structural characters of 

 the two species at once distinguishes them as not only 

 separate species, but also as belonging to different 

 genera ; and in addition to the generic differences, the 

 spicula of the skeleton of P. radiosa are acuate, while 

 those of I. ferula are acerate. 



"The anatomical structures of I. ferula are exceed- 

 ingly simple, but beautifully regular. The scalarif orm 

 skeleton tissues are composed throughout the whole 

 structure of a unispiculous network of acerate spicula, 

 and it is only very rarely that two spicula occur toge- 

 ther in any part of the primary lines of the skeleton. 

 The secondary lines of the skeleton are each composed 

 of a single spiculum, and they are disposed at such 

 regular distances from, each other as to cause the 



