456 Mr. H. J. Carter on the 



tubular general form, quite sufficient to identify with the spe- 

 cimens just mentioned, — also large fragments of the other or 

 spiniferous species, represented by Dr. Bowerbank in his 

 figure 1 (P. Z. S. I. c), which will be described under the name 

 of u Farrea densa " by-and-by. 



Up to the present time, then, this is all we know of Farrea 

 occa ; but as there have been several specimens of its deciduous 

 skeleton brought to notice, as well as many of Aphro- 

 callistes Bocagei, with the so-called " fistulose " character in 

 the fibre of all, it was not safe to state that all did not belong 

 to the same genus, viz. Farrea. Under these circumstances 

 there would be no hope of solving the problem, had it not 

 been found that, although in the fresh and living state of the 

 sponge hardly any trace of the spicules in the vitreous fibre 

 can be seen, yet after death a process of absorption takes place 

 in the interior of the fibre, whereby, if it has not gone too far, 

 the whole of the forms of these spicules may be recognized. 

 Hence, if any peculiarly characteristic spicules should happen 

 to be present in this fibre, the species of the sponge to which 

 it belonged can be determined, as in the case of the Aphro- 

 callistidge ; while in Farrea occa, where we have never had 

 any thing but the bare deciduous skeleton, the spicules which 

 it possessed in the living state might, under the same circum- 

 stances, be also discovered. It is to this process of absorption 

 and its effects in the sponge-spicule as well as the vitreous 

 fibre that we shall now more particularly direct our at- 

 tention. 



Taking first the siliceous sponge-spicule by itself, we find 

 that it is subject to two kinds of wasting or decay, viz. one 

 which takes place in the interior or wall of the central canal, 

 and the other on the surface — the former frequently occur- 

 ring in the living sponge, and the latter in the substance of 

 the spicule after death. 



The wasting which takes place in the wall of the central 

 canal is recognized by its increasing size, which in some cases 

 goes on until the spicule is reduced to a mere shell ; or it may 

 take place only at the ends of the spicule, when the central 

 canal at these points presents a funnel-shaped cavity diminish- 

 ing inwards or towards the centre of the spicule. In either 

 case the cause is not apparent. As this occurs in the living 

 state it is just possible that the central canal of the spicule, 

 which begins in a simple cell, may sometimes become so 

 dilated as to assume the form of a full-grown spicule, with 

 little if any vitrification, and thus appear as the mere shell. 

 To this may be added a general absorption of the proper spi- 

 cule, which frequently takes place within the horny fibre of 



