THE INVERTEBRATES 



133 



islands on which men live; others live parasitically in our 

 bodies to our great discomfort and danger; many are per- 

 sistent enemies of our crops and domestic animals. Finally, 

 all in their structure, their physiology, their development 

 and growth, their extraordinary adaptations to the con- 

 ditions of their life, their marvelous modes of distribution, 

 their beauty of color and pattern, and symmetry of outline, 

 appeal to that inborn love of knowledge in us, as subjects to 

 study, admire and enjoy. 



Sponges. — ^A bath or slate sponge is simply the skeleton, or 

 part of it, of a sponge 

 animal. In life all 

 of this skeleton is in- 

 closed or covered by 

 a soft, tough mass 

 of sponge flesh. 

 Sponges are fixed, ex- 

 cept when very young, 

 when they swim frpely 

 about. They are 

 found at all depths 

 and in all seas, grow- 

 ing especially abun- 

 dantly in the Atlan- 

 tic Ocean and the 

 Mediterranean. A 

 very few kinds live in 

 fresh water, being found in lakes, rivers, and canals, in 

 all parts of the world. The shape of the simplest sponges 

 is that of a small vase, or nearly cylindrical cup, attached 

 at its base, and having at the free end a large' opening 

 (fig. 53). But most sponges are very unsymmetrical and 

 grow more like a low, compact, bushy plant than like the 

 animals we are familiar with. The smallest sponges are 

 only I mm. (1-25 in.) high, while the largest may be over 



Fig. S3. A simple sponge, Granlia sp.; 

 at right a longitudinal section, showing 

 the simple body-cavity. (One-half natu- 

 ral size; after Jordan and Kellogg.) 



