NULLIPORHS. ]07 



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 zodthome, 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 Alga? 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. 



