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The graptolites built many theca together to form a branching structure that then drifted in ancient seas and therefore can be found in sedimentary rocks of a certain vintage all around the world.

This theca grows in the direction of the apex of the sicula, to which it adheres by its dorsal wall.

Before the latter plates arose, the stem had developed by the elongation and constriction of the fixed end of the theca, the gradual regularization of the plates involved, and their coalescence into rings.

Corals have been divided into Aporosa and Perforata, according as the theca and septa are compact and solid, or are perforated by pores containing canals lined by endoderm.

When the theca of the cord is directly infected the spinal symptoms predominate at first, but as the condition progresses it involves the cerebral membranes, and symptoms of acute general lepto-meningitis ensue.