208 University of California Publications in Zoology. [Vol.6 



to the number of canals, but there are certain exceptions. The 

 numbers always correspond in regenerating stems of small size. 

 but in very large stems two tentacles may be found occasionally 

 in our canal. The explanation of this appears to lie in the fact 

 that the canals are larger on the large stems than on the small. 



Pig. :i. — Transverse section of a portion of the wall of a hvdranth de- 

 veloping from a stem of moderate diameter. X 60. 



Fig. 4. — Beginnings of proximal tentacles, one to a canal; zone of frus- 

 tules below: somewhat contracted. X 9. 



There is a lower limit of size below which no canals are present. 

 There is also a minimum limit to the size of the canals, reached 

 when the stem possesses six to nine only. From this point the 

 number of canals increases with the diameter of the stem; hut 

 there is also an increase in the diameter of each canal. In very 

 large steins, the canals become exceptionally large, and appar- 

 ently afford room for more than one tentacle on each. The 

 number of canals may thus govern to a certain extent the number 

 of tentacles appearing in the first proximal set by growing in 



