CONTRIBUTION TO AN EVALUATION OF THE SEA. 313 



ing the various kinds of animals growing on them. Other experiments, 

 however, are being carried out with some success with the object of 

 entrapping young or larval free-living animals and observing their rate 

 of growth. The sexual condition of the collected animals has been 

 noticed particularly, and evidence of the maturity of the sexual products 

 obtained either by isolating the animals or by trying artificial fertilisa- 

 tions. As a result of these investigations a good deal of information 

 about members of most of the groups of invertebrate animals has been 

 obtained, and the following statements may be regarded as a sample of 

 the results. 



KATE OF GEOWTH IN SOME CCELENTERATA. 



Among the Coelenterata it has been found that various species of 

 Obeha and Clytia Johnstoni give off medusae when not more than a month 

 old. Similarly, Bougainvillea ramosa yielded medusae at once when 

 collected at an age of not more than six weeks and a few days. In all 

 these cases, however, the observations do not cover the whole life-cycle 

 as do the following. The commonest species of Tubularia — almost 

 certainly T. larynx — ^has been found to give off actinulae larvae at an 

 age of not more than six weeks, and in one experiment this species had 

 moderately developed gonophores at an age of not more than 26 days. 

 As these Tubularia actinulae have been found to settle and grow into little 

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

 life-cycle within about six weeks and probably actually within four or five 

 under favourable conditions. In the same way Plumularia and Gono- 

 thyraea have been found to give of! planulae at an age of not more than 

 three months ; thus, as these planulae may probably settle almost imme- 

 diately, these hydroids may complete their life-cycle within at least 

 three months, and in both cases in a period probably less than that stated 

 by some few weeks. Hence there can be little doubt that these hydroids 

 pass through several generations — probably three and possibly more — 

 in a year. A species of Syncoryne yielded medusae at a maximum 

 age of ten weeks, but doubtless the actual age was much less than 

 this. 



In all these cases, however, there can be little doubt that the rate of 

 growth — probably apart from the question of food-supply — varies at 

 different times of the year. So far as the investigations have been 

 examined, it would appear that in this group most of the species men- 

 tioned above appear to have a maximum rate of growth in the months 

 of AugTist and September. 



