114 



THE MECHANICAL FUNCTIONS. 



plants, but in their internal organization they differ entirely 

 from every vegetable production; being composed of a soft 

 flesh, intermixed with a tissue of fibres, some of which are 

 solid, others tubular; and the whole being interwoven to- 

 gether into a curious and complicated net-work. The sub- 

 stance of which this solid portion, or basis, is formed, is 

 composed partly of Jiorn, and partly of siliceous or calcare- 

 ous 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. 



The material of which the fleshy portion is composed is 

 of so tender and gelatinous a nature that the slightest pres- 

 sure is sufficient to tear it asunder, and allow the fluid parts 

 to escape; and the whole soon melts away into a thin oily 

 liquid. When examined with the microscope the soft flesh 

 is seen to contain a great number of minute grains, dissemi- 

 nated through a transparent jelly. Every part of the sur- 

 face of a living sponge (as may be seen in Fig. 53) presents 

 to the eye two kinds of orifices; the larger having a rounded 



55 



shape, and generally raised margins, which form projecting 

 papillse; the smaller being much more numerous, and ex- 

 ceedingly minute, and constituting what are termed the 

 pores of the sponge. 



It has, for a long time, been the received opinion among 

 naturalists that this superficial layer of gelatinous substance 

 was endowed with a considerable power of contractility: it 

 was generally believed that it shrunk from the touch, and 

 that visible tremulous motions coulcl be excited in it by 



