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the size of hydranths. This is clearly illustrated in the photographs 

 of Cordijlophora (Fig. 2) and Pcnnaria (Fig. 3); and both corre- 

 spond almost perfectly with the idealized pattern of Figure 1 A. 



Fig. 2. Pattern of a small colony of Cordylophora. 

 graph by Charles Wyttenbach. 



From a color photo- 



Figure 1 B illustrates the typical growth pattern found in many 

 thecate species. Growth zones give rise to the stems (pedicels) of 

 new hydranths but do not add to the length of the stem itself. 

 Hence the order of the age of the hydranths (in a young colony) is 

 from the base upward, 1, 2, etc., in Fig. 1 B. The youngest hy- 

 dranth is terminal— the opposite of the pattern in athecate species. 



In thecate species the pedicel of a new hydranth is completely 

 formed before the hydranth itself is produced. After the pedicel at- 

 tains its full length it enlarges at its tip to make a hydranth bud, 

 which then quickly differentiates into a hydranth. By the time the 

 hydranth emerges from its enclosing hydrotheca, it is fully func- 

 tional, and has its full set of tentacles and its full size. It grows 



