86 SYNOPTIC COLLECTION. 



the inorganic silica may have been replaced by the 

 organic horny material. The spicules vary but are built 

 on the triaxon plan. Besides the horny spicules there are 

 horny fibers which do not unite to form a network. 



We have here just those conditions which one might 

 expect to find in a transitional form between the silicious 

 and the horny sponges where the silica is replaced by 

 spongin, and where the horny skeleton has not yet become 

 the complex network seen in the more specialized genera. 



The canal system of Darwinella is simple and im- 

 branched and the ampullaceous sacs are of large size. 



Another genus, Aplysilla, is placed near Darwinella 

 which it resembles by having large ampullae, simple 

 canals, and isolated erect horny fibers, but it differs from 

 this genus by having no horny spicules. These have 

 wholly disappeared and the skeleton, now entirely fibrous, 

 is destined to develop in succeeding more specialized 

 forms until a labyrinthian network of fibers is the result. 



Hircinia campana Hyatt (No. 90), is normally vase- 

 shaped, but is subject to great variation, sometimes 

 becoming tubular, as proved by specimen No. 91. It has 

 been shown l that although this variation is great as 

 compared with the more specialized invertebrates, never- 

 theless a formula may be given which expresses the 

 possible range of variation in every species. One of the 

 simplest of the Hircinia (If. cactus) has a skeleton com- 

 posed of simple main fibers which contain foreign sub- 

 stance and slightly branched connecting fibers which are 

 free from foreign particles. The spongin of which the 

 fibers are composed is stratified and a granular axial 

 thread is present. 



In Verotigia fistnlaris Bon. (Nos. 92, 93), the fibers 

 are so large that their tubular form can be seen with the 

 eye. They are loosely put together, but the main and 



1 Hyatt, Mem. Boston Soc. Nat. Hist., II, pt IV, no. 5, 1877, p. 

 483- 



