VAETETT or ALECTOI^A MILL.VRT. 533 



presence of a dermal layer there. It seems probable therefore 

 that the growth of the sponge is limited by the shell externally. 



It is on examining the inner surface of the shell, however, that 

 the special growth-characteristics of the sponge are seen. Instead 

 of the smooth and even surface broken only by scattered perfor- 

 ations that is commonly seen in shells attacked by Cliona *, there 

 appears in this case an irregular elevated area covered with blunt 

 spines and papillse. 



Evidently the sponge has endeavoured to grow inwards, dis- 

 solving the nacreous layer and encroaching on the premises of 

 the mollusc, instead of restricting its wanderings to the thickness 

 of the shell. The mollusc has retaliated by depositing fresh shell- 

 layers on the intruder, and the struggle has gone on till the 

 chambers are several times the normal thickness of the shell, 

 and are roofed over by a thin convex layer of secondary shell- 

 substance, while the points at which branches have pushed further 

 in are represented by thick conical papillae. 



The actively growing parts of the sponge are fine threads, 

 sending off lateral branches or dividing dichotomously. 



A careful examination of the extremities of the borings indi- 

 cates that the excavation is carried on in the same manner as in 

 the Cliona described by Kassonow f. 



Immediately behind these growing tips the sponge thickens 

 rapidly, but does not form distinct chambers, so that there is 

 nothing of the moniliform appearance seen in some Clionas, 

 while the older parts occupy wide confluent spaces only crossed 

 here and there by pillars of shell. 



The rapidity with which the shell is attacked is shown by the 

 fact that only the extreme tips are in the normal thickness of the 

 shell, while the convex secondary deposit is developed almost to 

 the ends of the branches. 



The best idea of the relations existing between the sponge and 

 the shell is obtained from an examination of thin sections made 

 through the chambers and papillae. 



* Eyder in the ' American Naturalist,' 1879, vol. xiii. p. 281 , mentions the 

 presence of papillje on the inside of Ostrea virginiana peforated by a boring 

 sponge, but does not specify the sponge. I have seen small papillre due to 

 the presence of Cliona m My tiluslatus ; and Hancock mentions "clusters of 

 pearl-like points" due to Thoosa cactoides, Ann. &Mag. Nat. Hist. (2)iii. 1849, 

 p. 346. 



+ Zeitschrift fiir wiss. Zoologie, xxxix. pi. xviii. fig. 1. 



38* 



