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 insinuates 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 in a still clearer 

 light we have only to examine in detail any one species : we will 

 take C. gorgonioideSy PI. 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 

 adjoining 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 Cliona 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, PI. XIII. fig. I, and the regularly anastomosing and lobed 

 chambers filled by its branches ? How are the arborescent 

 channels and quadrate chambers of C. corallinoides, PI. XV. fig. 1, 

 formed ? and what excavated the numerous, regular chambers, 

 with their pointed spine-like ofi'sets, of C spinosa, PI. 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, 



