534 MK. A. y. JEKNINGS ON A 



Thus the section figured (PI. XIII. fig. 6) shows that while the 

 outer layer of the shell is left intact, the chamber occupied by the 

 sponge is fully twice the normal shell-thickness. The nacreous 

 layer is normally about -75 millim. thick; over the chamber it is 

 reduced to '40 millim., excex^t where the sponge has sent out 

 two branches that have been covered by many concentric deposits 

 to a thickness of 1-2 millim. 



In the axis of each papilla thus formed lie spicules of the 

 sponge, a pair of the large skeleton-spicules in one case lying 

 close together parallel to the direction of growth and surrounded 

 by the small flesh-spicules. The appearance of these sharp and 

 spinose bodies lying in a crypt scarcely larger than themselves, 

 like a Pholas in its burrow, might have been used as evidence in 

 favour of the view that the excavating power of boring-sponges 

 is due to the spicules. 



One feature of considerable interest remains to be noted. It 

 is only in quite the youngest regions that the sponge has been 

 able to establish communication with the exterior on the inside of 

 the shell. In all the older parts its attempt to grow inward, and 

 the consequent deposit of shell over it, has prevented the for- 

 mation of apertures. All the papillae are closed at the apex, 

 showing that the mollusc can deposit new shell faster than the 

 sponge can dissolve it. 



In the case of Gliona, there has been mucli diflFerence of opinion 

 as to the effect on one another of the two organisms. Thus 

 while some writers have held that the sponge commonly dies 

 first, Hancock's * opinion w^as that the death of the sponge came 

 only on the breaking up of the shell, when, " Samson like, it 

 perishes amidst the ruin produced by its own energy." 



Whatever be the case with Cliona, it is evident that the sponge 

 has not the best of the struggle in the present case, and it seems 

 probable that the species is in a transition condition, and repre- 

 sents one of the stages in the evolution of shell-boring Porif era. 



The original type inhabited a coral, in which it could grow 

 freely in all directions. In the present instance a similar irre- 

 gular growth has been most injurious if not fatal to the sponge, 

 owing to waste of energy in dissolving shell that can be readily 

 replaced, and to the deficient circulation resulting from the 

 closure of apertures. 



* Hancock, Ami. & Mag, Nat. Hist. iii. 1849, p. 323, 



