BY SPONGES OF FOREIGN SPICULES. 151 



no aualogous facts in confirmation, the evidence rather tending 

 to show that as growth continues the surface-spicules, if of the 

 same kind as those of the main skeleton, become incorporated 

 with it either by adhesion to the perpendicular (primary) lines or 

 by remaining horizontal to form the latest- formed " secondary 

 lines" ; at any rate, the appearances seen in perpendicular sections 

 of some of the regular Isodictya seem to point to this conclusion. 



The other instance is that of Alebion (Grray) sp., and needs no 

 special detailed account, as the circumstance of occurrence of the 

 foreign spicules is essentially the same. It is also the same spi- 

 cule (measuring here slightly less, *15 by "01056 to -01108 millim.) 

 which is the intruder. It occurs scattered or in bundles in the 

 dermis. The sponge is also from the same locality and depth as 

 the Esperia above mentioned ; but, unfortunately, the number of 

 the jar in which it arrived is not preserved, so that it cannot be 

 presumed with such probability that it is from the same haul of 

 the dredge. 



It only remains to draw the attention of workers at sponges to 

 these two cases, by way of warning against being misled by spi- 

 cules occurring chiefly in the dermis of sponges, which are not so 

 universally distributed there as to lead to the conclusion that they 

 belong to the sponge, even though, as in this case, the fine preser- 

 vation of the spicule and absence of enlargement of the central 

 canal would not suggest their being foreign to it. Sometimes this 

 may be seen at a glance, as when an obviously calcareous triradiate 

 spicule is found in a sponge whose complement is made up (e. g.~) 

 of siliceous acerates, or when an unmistakable Geodia-b&ll occurs 

 under similar circumstances. But the present is one which 

 differs essentially from such cases ; the spicules are well pre- 

 served, present some degree of regular arrangement, and are 

 not radically distinct from the type which would be expected in 

 the sponge. Still the difference of form and the manner of 

 occurrence are sufficient to point out their foreign origin in 

 this case ; and if, as is far the most usual, they had been 

 broken, or their central canals were enlarged by absorption, or 

 the heads had projected outwards and the points inwards, these, 

 which are the safest proofs of the foreign origin of a spicule, would 

 have infallibly guided to a correct judgment as to their nature. 



