GROWTH A N D LIFE HISTORIES 67 



off at fairly regular intervals. For instance, a newly settled Balanns 

 amphritrite niveus usually moults on the fourth day after settling, 

 and follows this by moulting on the sixth, eighth and tenth days. 

 A total of 19 moults may be passed through during the first 47 

 days after settling. There does not seem to be anv connection 

 between the rate of growth of the outer shell and the moulting 

 cycle. 



As in all Crustacea the growth of barnacles is controlled bv 

 various environmental factors. One of the most important of these 

 factors is the length of time that the barnacle is left uncovered by 

 the tide. Total submergence appears to be the ideal state for most 

 barnacles, and it is found that their growth rate decreases with 

 increasing duration of exposure to the air. This results in barnacles 

 growing at different rates at different levels on the seashore; those 

 near the top of the shore grow more slowly than those which are 

 lower down and less frequently uncovered. 



The rate of flow of water past a barnacle also influences growth. 

 Water currents up to 1-5 knots actually increase the rate of growth, 

 probably by bringing more food within reach. Faster currents cause 

 a decrease in growth rate, and dislodge young barnacles. This is 

 important in relation to barnacle-fouling of ships. If only short 

 stays are made in port there will be very little permanent fouling 

 because the young barnacles are dislodged when the ship starts 

 moving. 



There seems to be a certain minimum size, characteristic for each 

 species, which must be reached before a crustacean becomes 

 sexually mature. Thus a female Daphnia magna does not reproduce 

 before she reaches a length of 2-1 mm., while a female lobster 

 reaches a length of ten inches before she starts breeding. Matura- 

 tion is, of course, characterised bv an increase in size of the gonads 

 and the production of germ cells. Hormones are believed to control 

 these processes, and experiments indicate that in some species the 

 eyestalks produce a substance which inhibits the growth of the 

 gonads, and that the Y-organ is necessary for sexual maturation, 

 but not necessary for the maintenance or* sexual functions once the 

 animal has become mature. 



The gonads themselves may act as endocrine organs and produce 

 substances which are necessary for the production of secondary 

 sexual characteristics. The testes of isopods produce such a 

 hormone. In the amphipods there is a special gland on the male 

 duct which seems to be responsible for maintaining the secondary 



