with descriptions of new Species. 325 



is perhaps the most remarkable : in this the terminations of the 

 branches have a decided dendritic appearance; they divide pretty 

 regularly into two portions, which send off on all sides numerous, 

 minute, linear twigs. The two principal portions soon swell out 

 and form into oval lobes similar to those of the stems, which are 

 seen likewise to give off linear twigs. These twigs and those of 

 the terminal branches are so much alike, that it is impossible to 

 doubt that the lobes of the stems have at one time been them- 

 selves terminal branches. 



Thus is the form of the excavating sponges varied, and the 

 chambers they inhabit modified; each species being always found 

 in the same-formed cavities ; that is, those with the same kind 

 of spicula, and with papillae of the same size, number and ar- 

 rangement, are always found to branch and to anastomose in a 

 similar manner, and to have the terminal twigs of the same cha- 

 racter. This surely could not happen, did Cliona take up its 

 abode in cavities caused by decay, or in excavations formed by 

 worms, and were its shape dependent upon such accidental cir- 

 cumstances. 



The cavities in shells occupied by Cliona have at once, without 

 examination, been attributed to worms ; and as inquiry was thus 

 easily satisfied, the matter has remained up to tlie present time 

 in obscurity. Those naturalists, however, who have paid parti- 

 cular attention to the subject appear inclined to a contrary opi- 

 nion. Professor Grant says, the chambers " have probably been 

 perforated by some worms;" though at the conclusion of his 

 communication before alluded to on the subject, it is stated that 

 " it may be questioned whether the sharp siliceous spicula and 

 constant currents of its papillae do not exert some influence in 

 forming or enlarging the habitation of this zoophyte." By 

 Johnston's work on the British sponges it also appears that 

 Mr. Wm, M'Calla, who found Cliona celata in Birterbuy bay, 

 states that this animal "is very destructive to the shells that 

 come within its reach," and that in several instances " he had 

 found large specimens of Pecten operculaj'is killed by the en- 

 croachments of this parasite." And so satisfied was M. Duver- 

 noy that the species described by him excavated its own habi- 

 tation, that he gave to it the specific denomination of terebrans. 

 The prevailing belief is as before stated, however, that Cliona 

 does not excavate the chambers in which it is found ; but that 

 they are formed by worms or by decay, or are produced in some 

 other accidental manner ; and that the shape of the sponge 

 depends on that of the cavities it may chance to inhabit. 

 Were this belief correct, the chambers would occasionally occur 

 only partially occupied. This never happens ; for Cliona always 

 completely tills the various chambers and ramifications even to 



