REPRODUCTION AND GROWTH 261 



a conspicuous hydrosome stage by H, an inconspicuous or 

 degenerate hydrosome stage by h, and an inconspicuous or 

 degenerate medusome stage by m. 



1. O — H — O — {Hydra) Fresh-water. 



2. O — H — m — O {Sertularia) \j -.. 1 



3. O-H-M-0 {Ohelia) /^^"°^^^' 



4. O-h-M-0 {Liriope) ) j^.^.^.^ 



5. 0--M— O {Geryonia) ] ^^^P ^^^' 



From the above table it would appear that, on the whole, 

 the disappearance of the medusoid stage is associated with 

 littoral or fresh- water conditions, and that of the hydroid 

 stage with conditions of life in the deep sea. Plausible 

 enough as this explanation seems it cannot be accepted with- 

 out qualification, since of two closely related forms like 

 Ohelia geniculata and Campanularta flexuosa living under 

 apparently quite similar conditions, one bears sporosacs and 

 the other true medusoids (Newbigin, 19 10). The fact 

 of the matter is that the feature of the shore as essentially 

 a transitional area is reflected in the varied means by which 

 shore animals secure survival. 



With regard to the growth rate of Hydrozoa, Or ton {op. 

 cit.) finds that many species of Ccelentera give off medusae 

 when not more than a month old. A Tubularian (pro- 

 bably Tuhiilaria larynx) was found to give off actinula larvae 

 at an age of six weeks, and in one experiment this form had 

 moderately developed gonophores in twenty-six days. As 

 these actinulae have been found to settle and to grow into 

 polyps within a few days it follows that this species may pass 

 through its life-cycle in six weeks and, if conditions are 

 favourable, a week or fortnight sooner. A raft moored at 

 sea for six weeks was found to be covered with a growth of 

 Ohelia geniculata. In Plumularia and Gonothyraa the life- 

 cycle may be completed in three months and probably more 

 quickly still if conditions are favourable. Hence there is little 

 doubt that these hydroids pass through several generations 

 in a year. Most of the Coelenterate species investigated by 

 Orton appear to have a maximum rate of growth in the 

 months of August and September. 



