species, which is flat or chubby in (juiet water, will tend to devolop into branching 

 forms. 



This plasticity of fonu in response to environment also correlates with the pecu- 

 liarities of the digestive system. The sponges have thousands of minute cavities 

 within the body, devoted to performing the functions of digestion. These cavities re- 

 ceive their food from streams of water, circulating through a double system of tubes, 

 and flowing in through the narrow meshes of a network, formed in the outer covering 

 or skin of the body. Witli this sieve-like structure there is no use for any jjarticular 

 set of external appehdages, and no necessity for any fixed symmetry of form. All that 

 the sjionge needs is a capability to adapt itself to its surroundings and the sole 

 requisite of success in obtaining food is the presentation of as much surface as possible, 

 tiuis securing a large supply of water and accompanying food. 



Such an organism requires a peculiar skeleton. Since the internal tubes and mi- 

 nute stomachs would be liable to compression by the weight of the soft tissues, after 

 the attainment of a certain size, unless some firmer framework was interposed, we find 



Fig. 48. — Portion of a section of a bath-sponge (Sjtont/ia), showing the fibrous skeleton, portions of the 

 supply and drainage systems, and the aiupuUse. 



in most sponges such a supporting skeleton. In some cases this framework is formed 

 I ly a woven mass of elastic threads, of a horny nature; in others the framework is 

 composed partly of such threads and partly of stiff and nnelastic spicules which may 

 be calcareous or silicious, or in still other cases of a network of spicules united by only 

 a small amount of horny or silicious material. The same principle of construction runs 

 throughout the whole of the Porifer.ata; the skeletons are really networks or scaffolds 

 of spicules, or of threads permeating all ]iarts of the body, in order to sup])ort the 

 whole mass and keep open not only the digestive ampuUie, but also the numerous 

 tubes for supply and drainage. 



A skeleton is not, however, an absolute essential in all the members of anj' branch 

 of the animal kingdom ; thus there are sponges entirely destitute of spicules or threads, 

 but these are mostly flattened or small vase-like forms, in which the weight is small in 

 proportion to the strength of the tissues. 



In the commercial sponges the skeleton is an intricate mass of interwoven elastic 

 horny threads, as may be seen by slicing one through the middle (Fig. 48). This network 



