CNIDARIA 



67 



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lie at the lowermost end of the stem are the oldest. In 

 neai^ly all of the Caljcophoridne and some of the Physo- 

 phoridae (Apolemia) the individuals of the stem are 

 arranged in definite oroups (cormidia), which are separated 

 from one another by free portions of the stem (internodes) . 

 In many other forms, on the other hand, 

 the limits of the successive internodes are 

 indicated merely by the attachment of the 

 polypi tes with their tentacles (Fig. 29 A, 

 B, C, D), whereas the parts of the stem 

 lying between the polypites are occupied 

 by groups of individuals (consisting of hy- 

 drophyllia, dactylozooids, and gonophores). 

 (In the accompanying figui-e, for the sake 

 of simplicity, instead of these groups of 

 individuals, only their dactylozooids are 

 indicated.) Here the law that growth pro- 

 gresses unifoi*mly from above downwards 

 applies only to the polypites (A, B, G, B), 

 whereas each internode presents its own 

 zone of growth for the groups of indivi- 

 duals (a, 6, c, d) belonging to it, for which 

 in turn the uppermost end of the internode 

 must be looked upon as the budding point, 

 so that likewise in the series of groups of 

 individuals in each single internode the 

 lowermost (a) is the oldest. Each inter- 

 node of the stem is divided by these groups 

 of individuals into internodes of the second 

 order (.4a, ah, be, cd . . .); and each such 

 internode of the second order may, in the 

 further growth of the stem, become a zone 

 of growth for a series of new groups of in- 

 dividuals (a, 13, y . . .) (Chun, No. 57). 

 For the other groups [of Physophoridae] 

 the details of the laws of buddinsr are as 

 yet little known. In the VeUelidse the for- 

 mation of the individuals takes place in 

 concentrically arranged circles. 



/ 



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—a 



Fig. 29.— Diajjram 

 of Chun's law of 

 budding of the 

 groups of indivi- 

 duals on the stem 

 of Halistemma. In 

 place of the groups 

 of individuals only 

 the corresponding 

 dactylozooids are 

 shown. 



