104 MARINE INVERTEBRATES 



but if vertical growth is not greatly in excess and is restricted to the 

 marginal part, cup-shaped forms result, and tubular forms when 

 lateral growth is particularly restricted. In some varieties tubular 

 masses coalesce ; sometimes flat forms unite in intricate com- 

 pressed folds. Differences in rapidity of vertical growth make 

 undulations more or less marked, producing lobes and protuber- 

 ances. Sometimes narrow vertical growth is retarded and hori- 

 zontal growth predominates, forming various shapes on apparent 

 stems. 



Many of the horny sponges are colored, in shades of yellow, 

 brown, red, and violet ; some are black. 



Sponges are divided into two classes, the Calcarea and Non- 

 Calcarea. The former have calcareous skeletons, which make 

 them hard and rigid ; the latter have skeletons of spongin-flber 

 or of silicious spicules, or of the two combined. 



All sponges, with the exception of one family (the Spongillidce, 

 which live in fresh water), are inhabitants of the sea, and live at 

 various depths. The sponges of commerce belong to the Non- Cal- 

 carea and are all confined to the genus Spongia. In this genus the 

 skeleton is more or less flexible, being composed of spongin. 

 They are shallow-water species, are confined to seas where the 

 waters are of comparatively uniform high temperature, and 

 flourish best when protected by reefs and islands. 



There are fisheries for sponges in the waters of the West 

 Indies, the Bahamas, on the southern and western coasts of 

 Florida, and in the Mediterranean and Red seas. Those of 

 the Mediterranean surpass in quality the sponges of our 

 coasts. 



Five species of commercial sponges are taken from Florida 

 waters. They are graded by the trade in the order of their im- 

 portance, as the " sheepswool," " yellow," " grass," " velvet," and 

 " glove." The fishing for sponges is done from small boats, two 

 persons manning each boat. One man sculls, while the other, 

 using a water-glass, scans the bottom. The water-glass is a box 

 with a pane of glass on the bottom. If the glass is held below the 

 surface and the face is placed in the box, the observer is enabled 

 to see with sx>me distinctness the bottom at 9, considerable depth 



