212 On the Sjriral Character of Coelenterate Development. 



XVII. — The Sjriral Character of Coelenterate Development. 

 By Prof. John Young, University of Glasgow. 



In the Anthozoa fundamental numbers have been recognized — 

 four for the Ceriantlias group, six for the Stone Corals, 

 eight for the Alcyonarians. Mereschkowsky considers the 

 fundamental number for the Hydrozoa to be two (Ann. & 

 Mag. Nat. Hist. ser. 4, vol. xx. p. 220). Gegenbaur speaks 

 of the tentacles as varying so that " we cannot suppose that 

 the secondary axes are definitely differentiated." Hackel 

 refers the Coelenterate type to an imaginary six-sided pyra- 

 mid. Metschnikoff and Mereschkowsky have stated the mode 

 of development of the tentacles among the Hydroida in such 

 fashion as to give support to the view that the partition of the 

 Coelenterates may be explained by analogy of the floral axis 

 in phamogamous plants. The septa of the Anthozoa are de- 

 veloped in successive cycles, and symmetrically with reference 

 to the bilateral symmetry, of which the young and even the 

 adult stage give evidence. Huxley, following Lacaze-Duthiers, 

 gives ( l Manual of the Invertebrates,' p. 158) the following 

 formula for the chambers of the hexamerous Anthozoon : — 



.CE.FDB A , 



Mereschkowsky shows that the partition of the reproductive 

 sacs in the gonophore of Monobrachium takes place similarly 

 in paired fashion, the bilateral symmetry being there also 

 not lost sight of. As the later septa are successively shorter 

 than those immediately preceding, it appears as if the radiate 

 appearance of the Coelenterata were due, not to the production 

 of equivalent antimeres round a common centre, but to the 

 shortening of an axis which had given off successive pairs of 

 buds, the last pairs being represented on the shortened axis by 

 the septa of least development. Whether the shortened axis 

 is raceme or corymb depends on the development of the repro- 

 ductive organs, on the order in which they reach maturity on 

 the free margin of the mesenteries. The formula quoted 

 above seems to be the expression of the spiral development. 

 Any one who follows the development of the tentacles of a 

 well-fed Hydra may see that the order of their appearance 

 is not irregular, but that it follows a definite rule. No doubt 

 the view here proposed has occurred to other anatomists ; but 

 as it has not, to my knowledge, been published, I now seek to 

 put it on record as a protest, were it nothing more, against 

 the geometric method of classifying animal forms. 



