NULLIPORES. 107 
distinctly. The animals, unlike true polyps and the Hydroids, 
have two extremities to the alimentary canal, and in this, and 
other points, they are Molluscan in type. 
The cells of a group never have connection with a common 
tube, as in the Hydroids; on the contrary, each little Bryozoum, 
in the compound group or zoédthome, is wholly independent of 
the rest in its alimentary canal. 
Bryozoans occur in all seas and at all depths; and in early 
Paleozoic time they contributed largely to the making of lime- 
stone strata, 
IV. NULLIPORES. 
The more important species of the Vegetable Kingdom that 
afford stony material for coral reefs are called Nullipores. 
They are true Algze or sea-weeds, although so completely stony 
and solid that nothing in their aspect is plant-like. They form 
thick, or thin, stony incrustations over surfaces of dead corals, 
or coral rock, occasionally knobby or branching, and often 
spreading lichen-like. 
They have the aspect of ordinary coral, especially the Mil- 
lepores, but may be distinguished from these species by their 
having no cells, not even any of the pin-punctures of those 
species. 
Besides the more stony kinds, there are delicate species, of- 
ten jointed, called Corallines, which secrete only a little lime in 
their tissues, and have a more plant-like look. Even these 
grow so abundantly on some coasts, that, when broken up and 
accumulated along the shore by the sea, they may make thick 
calcareous deposits. Agassiz has described such beds as hav- 
ing considerable extent in the Florida seas. 
