OF THE SPONGIADJB. 197 



number of spicula of which their diameter is formed, some- 

 times consisting of two or three spicula only, and at other 

 times of more than it is possible to count. They often 

 divide, the branches passing in different directions, but they 

 never reunite or anastomose with other fasciculi. A portion 

 of this network of spicula is represented by Pig. 375, Plate 

 XXXV. The columnar axis of the cioacal system consists 

 of one large spiral fasciculus of spicula, each of which 

 extends from the base or very near that part of the sponge, 

 to near or quite to the apex of the column, the direction of 

 the spiral being from right to left. Pig. 374, Plate 

 XXXV, represents a portion of the great cioacal column, 

 exhibiting part of the spiral axial fasciculus surrounded by 

 the remains of its dermal coat, with numerous oscula pro- 

 jecting from its surface, copied from ' Zoological Proceed- 

 ings ' for 1857. 



There is a close approximate alliance to this form of the 

 cioacal appendage of Hyalonema in the corresponding organs 

 of the British genus Ciocalypta, Bowerbank. 



IsoDiCTYA, Bowerbank. 



Spoitgia, Montagu. 

 Haiichondma, Fleming. 

 Hamchondhia, Johnston. 



Skeleton without fibre ; composed of a symmetrical network 

 of spicula ; the primary lines of the skeleton passing 

 from the base or centre to the surface, and the 

 secondary lines disposed at about right angles to the 

 primary ones. Propagation by internal, membra- 

 naceous, aspiculous gemmules. 



Types, Isodictya palmata and Normani, Bowerbank. 



This genus, in the structure and arrangement of its 

 skeleton, is intermediate between Halichondria and Chalina, 

 as defined in the present work. Like the former, the 

 spicula of the network composing the skeleton are merely 

 cemented together, not inclosed within a regular horny 



