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 mpossible 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 mbabit modified ; each species being always found 
in the same-formed cavities; that is, those with the same kind 
of spicula, and with papille of the same size, number and ar- 
rangement, are always found to branch and to anastcmose 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 im 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 Ciona have at once, without 
examination, been attributed to worms; and as inquiry was thus 
easily satisfied, the matter has remained up to the present time 
in obseurity. ‘Those naturalists, however, who have paid parti- 
eular 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 
eommunication before alluded to on the subject, it is stated that 
“it may be questioned whether the sharp siliceous spicula and 
constant currents of its papille 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 opercularis killed by the en- 
eroachments 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 speeifie 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 fills the various chambers and ramifications even to 
