SYSTEMATIC ZOOLOGY FOR TEACHERS. 



219 



Leaving the upright forms of sponges for a moment, let us try and 

 trace sponges from their origin. Encrusting stones, dead coral, etc. in tropi- 

 cal waters, we find protoplasmic-like masses which, when we come to study 

 their structure, we find are really sponges, but in the cases which we have 

 under consideration, are wholly without fiber or spicules of any kind. This 

 group of sponges is known as the fleshy sponges, and in some such struct- 

 ures, sponges must have originated. That is, sponges with a protoplasmic 

 structure must have been the originators of sponges with both horny fiber and 

 spicules. As they exist at present these fleshy sponges have a rather com- 

 plicated water system. A figure of this water system is given in fig. 107. 



Fig. 101 



Fig. 102. 



Logger-head Sponge. 



Scarlet Sponge. 



It can readily be seen that what was at first some accidental accumula- 

 tion of either spicules or of horny matter, proved of such benefit to the fleshy 

 sponge, that it became an inherited habit with these sponges to secrete either 

 fiber or spicules. A careful study of the species of sponges would tend to 

 show that spicules were the original supporting skeleton of sponges, and that 

 horny fiber appeared afterwards. 



While there is some evidence to show that these spicules may have 

 formed first around the excurrent tubes, thus causing them to be projected 

 upwards in the form of comparatively narrow tubes, it may be possible that 

 ' wide disks were formed around cup-shaped depressions, and in place of 

 the tubes evolving cups, the cups themselves may have evolved the tubes. 

 The constant inclination of the finger sponge to produce cups, shows a tenden- 

 cy in this species to revert to an original cup-like form. On the other hand 

 we find ( as will be seen later ) that the tube sponge shows decided inclina- 

 tion to produce narrow tubes, or filamentous growths, indicating that it had 



