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 Calearea and Non- 
Calearea. The former have calcareous skeletons, which make 
them hard and rigid; the latter have skeletons of spongin-fiber 
or of silicious spicules, or of the two combined. 
All sponges, with the exception of one family (the Spongillide, 
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 some distinctness the bottom at a considerable depth 
