OP THE SPONGIAD^. 135 



acuate entirely and verticillately spined defensive spicula in 

 situ, represented by Figs. 289, 290, Plate XVII, one 

 of them has the spicula collected in groups in a manner 

 very similar to those of the spinalo-recurvo-quaternate 

 form, and if the latter be considered as organs for the 

 retention of prey, the physiological purpose of the grouping 

 together of the former can scarcely be considered in any 

 other light. 



In the isolated positions of these forms of spicula, viewed 

 in reference to some ideas regarding their physiological pur- 

 poses, there are circumstances of a very remarkable nature. 

 These forms of spicula occur in several distinct genera of 

 sponges, and especially in those having a strong kerato- 

 fibrous skeleton. Their usual locality is on the fibre of the 

 -skeleton, in which their bases are firmly imbedded, and 

 from which they are projected at various angles into the 

 canals and cavities of the sponge, and they are very rarely 

 seen on the membranes. In HymerapMa stellifera (Fig. 

 370, a, Plate XXXIV) and H. davata, Bowerbank, both 

 exceedingly thin coating species, they occur in great quan- 

 tity, but only on the basal membrane ; a portion of them 

 being erect, the remainder prostrate. But in another 

 sponge, a remarkably curious parasitical species, Hymenia- 

 cidon Cliftoni, Bowerbank, MS. (Fig. 291, Plate XVII), 

 which having no fibrous skeleton of its own, covers and 

 appropriates a small fibrous Fucus, and converts its anasto- 

 mosing vegetable stalks into an artificial skeleton, closely 

 coating each stalk of the plant with its membranous struc- 

 ture, so as to cause them at first sight to be readily mis- 

 taken for keratose sponge fibre. The whole of the mem- 

 branous structure of this sponge abounds with attenuato- 

 cylindrical entirely spined defensive spicula, but they are 

 all prostrate and intermingled with the skeleton spicula of 

 the sponge when not in contact with any part of the fibres 

 of the vegetable, but wherever they are in contact with the 

 plant they instinctively, as it were, assume the erect posi- 

 tion, and the false skeleton is bristling with them to as 

 great an extent as if it were truly a kerato-fibrous struc- 

 ture. This feature in the habit of the sponge is very 



