SPONGES. 135 



modern writers, would appear to be a more appro- 

 private designation. It is, however, scarcely worth 

 while, at the present day, to change a name so 

 generally received as that of Zoophytes, and the 

 application of which is not likely to lead to any 

 misunderstanding. 



<§ 2. Sponges. 



Among Zoophytes, the lowest station in the scale 

 of organization is occupied by the animals which 

 fabricate the various species of sponge, and which 

 are met with in such multitudes on every rocky 

 coast of the ocean, from the shores of Greenland to 

 those of Australia. Sponges grow to a larger size 

 within the tropics, and are found to be more dimi- 

 nutive, and of a firmer texture, as we approach the 

 Polar circles. They adhere to, and spread over 

 the surface of rocks and marine animals, to which 

 they are so firmly attached that they cannot be 

 removed without lacerating their bodies. In their 

 general appearance they resemble many kinds of 

 plants; but in their internal organization they differ 

 entirely from every vegetable production ; being 

 composed of a soft flesh, intermixed with a tissue 

 of tubular fibres ; and the whole being interwoven 

 together into a curious and complicated net-work. 

 The substance of which this solid portion, or basis, 

 is formed, is composed partly of horn, and partly 

 of siliceous or calcareous matter. It has been termed 

 the axis of the Zoophyte ; and as it supports the 

 softer substance of the animal, it may be regarded 

 as performing the office of a skeleton, giving form 

 and protection to the entire fabric. 



