2 20 THE AMERICAN MUSEUM JOURNAL 



tube-like forms, and even the simplest and most primitive 

 sponges are often shaped like graceful vases. All these forms are 

 found in sizes varying from that of a pinhead to the height of 

 a man. 



Their color is as varied as their shape and size. They rim 

 through the whole chromatic scale from brilliant red, yellow, and 

 green to the most delicate blue and the deepest violet, in every 

 gradation of shade and tint. Some are pure white, others are 

 shining black, while still others reflect from their opal spicules 

 all the colors of the rainbow. 



As the form and color of sponges, however, may vary as 

 much among members of the same species as among those of 

 different species these factors cannot be depended on for classi- 

 fication. The same sponge which in deep water shows the branch- 

 ing habit, in shallower water appears as a flat encrusting colony; 

 or a sponge which has a symmetrical vase-like form, many feet 

 below the surface of the sea, where it is little disturbed by outer 

 influences, may be of the same species as an irregular one-sided 

 mass growing in shallow water or in the crevice of a rock. Again 

 a sponge usually dome-shaped may send out a finger-like process 

 from its upper surface which becomes branched and unites with 

 the branches of other finger-like processes. In other words 

 external form in sponges is not a constant or essential factor. 

 It is purely a matter of environment, in which gravity plays an 

 important part. This tendency to vary has made the arrange- 

 ment of sponges in an orderly and natural system, a difficult 

 task, much complicated by the fact that for many years classifica- 

 tion has been wrongly based upon these very factors. Since, 

 however, the microscope has been developed to its present 

 perfection, it has been found that the arrangement of the skeleton 

 and the form of the spicules or skeletal units, together with the 

 structure of the canal system, furnish more constant data for 

 classification. This can be brought out more clearly in dis- 

 cussing the anatomy of the sponge. 



ANATOMY. 



In considering tlic anatomy of sponges it is sufficient for our 

 purpose to concern ourseh'cs with : 



