OF THE SPONGIADyE. 31 



unnecessary length by describing every mode of their 

 occurrence, but select a few of the most interesting 

 cases as illustrations of the general principles of their 

 application. 



Fig. 288, Plate XVII, represents a small portion of 

 the kerato-fibrous skeleton of an Australian sponge, with 

 the attenuato-acuate entirely spinecl internal defensive 

 spicula in situ. Fig. 289, represents a few fibres from a 

 kerato-fibrous sponge from the West Indies, in which the 

 vertici] lately spined internal defensive spicula are dispersed 

 over the fibres ; and Fig. 290 represents the same descrip- 

 tion of defensive spicula from a West Indian kerato- 

 fibrous sponge, having the defensive spicula congregated in 

 bundles. Sometimes, but not very frequently, they are 

 found on the interstitial or basal membranes of the sponge, 

 and under these circumstances many of them are prostrate 

 in place of being erect ; and in one sponge, Hymeniacidon 

 CUftoni, Bowerbank, MS., a singular parasitical species 

 from Freemantle, Australia, this prostration appears to be 

 effected by an especial law. This singular sponge enve- 

 lopes several fan-shaped portions of a Fucus, and syste- 

 matically appropriates the minute ramifications of its stem 

 to the purposes of an artificial skeleton ; the whole sponge 

 abounds with short stout attenuato-cylindrical entirely 

 spined internal defensive spicula ; but the remarkable 

 circumstance attendant on their presence is, that where- 

 ever the membranes supporting them envelope and firmly 

 embraces a portion of the vegetable stem, they assume an 

 erect position, and exhibit all the usual characters of 

 defensive spicula ; but where the membranes merely fill 

 up the areas of the vegetable network, they are nearly 

 all of them perfectly prostrate and apparently performing 

 the office of tension, rather than of internal defensive 

 spicula. Their form also is singular, being attenuato- 

 cylindrical. not having the acute termination that is usual 

 in this description of spicula. 



Fig. 291, Plate XVII, represents a small portion of the 

 fibrous stem of the Fucus coated by the membranes of the 

 sponge, and covered with spicula ; those immediately over 



