326 Mr. A. Hancock on the Excavating Powers of Sponges, 
the end of the most minute twigs. ‘To this we have the testimony 
of Professor Grant, who says that the form of Cliona “ depends 
on that of the cavities which it fills; it msinuates itself into the 
minutest ramifications, and adheres so closely to their smooth pa-~ 
rietes, that it cannot be separated without tearing.” 
From what has already been said respecting the form and 
mode of growth of these sponges, it is pretty evident that they 
must form their own habitation. But to put this im a still clearer 
light we haye only to examine in detail any one species: we will 
take C. gorgonioides, P|. XIV. fig. 1, as an example. The prin- 
cipal stems of this species take a zigzag direction, sending off at 
the angles lateral branches which pass on to unite with the neigh- 
bouring stems : the terminal twigs are delicate and bifurcate, one 
of the divisions going immediately to form its junction with the 
adjoming stem. This mode of growth goes on until the entire 
substance of the shell in which the Cliona is lodged is completely 
filled with a network of branches ; the anastomosing increasing 
all the while by the addition of twigs from the main stems until 
very little of the shell is left to separate the various parts of the 
sponge. Now in all this there is nothing having the appearance 
of accident. Where the Ciona is not, the shell is perfectly sound 
and untouched: the terminal twigs are all alike delicate and of 
similar character, penetrating the hard perfect substance; the 
main stems become gradually and proportionately thick, and the 
anastomosis, though somewhat irregular, is identical throughout. 
And this takes place whether the specimen is buried in Fusus, 
in Buccinum, in Ostrea, or in limestone, in all of which this spe- 
cies occurs. 
Now if we assume for a moment that these sponges are in- 
capable of excavating the chambers in which they conceal them- 
selves, how shall we account for the formation of the beautiful 
dendritic cavities occupied by the terminal twigs of Thoosa cac- 
toides, P|. XIII. fig. 1, and the regularly anastomosing and lobed 
chambers filled by its branches ? How are the arborescent 
channels and quadrate chambers of C. corallinoides, Pl. XV. fig. 1, 
formed ? and what excavated the numerous, regular chambers, 
with their pointed spine-like offsets, of C. spinosa, Pl. XIII. fig. 5? 
How shall we answer these questions, unless we can assert that 
the sponge inhabiting those systematic cavities formed them ? 
They are evidently not the result of decay, neither are they the 
burrows of worms ; which, when in shell or other hard calcareous 
substance, are always linear, sometimes cylindrical, often de- 
pressed, never lobed, and frequently double, that is, with two 
channels divided by an elevated ridge. And so different are they 
in their general appearance, that it is very easy to point out 
which is the excavation of the worm, and which that of Cliona, 
eet a ee le ee ee eT 
