GROWTH AND LIFE HISTORIES 6g 



need to be fertilised. Daphnia and most other Cladocera provide 

 good examples. In suitable conditions the females produce eggs 

 which give rise to other females without being fertilised; this 

 process may be repeated for hundreds of generations. In poor con- 

 ditions males appear among the offspring, and some of the females 

 produce eggs which need to be fertilised. These fertilised eggs 

 can withstand adverse conditions, such as drying, and serve to 

 carry the population through periods when environmental con- 

 ditions are difficult. They are often called resting eggs. Some 

 populations of Daphnia can produce resting eggs which are not 

 fertilised, and have completely dispensed with males. 



Some of the eggs produced by Anostraca and Conchostraca are 

 peculiar in shape, or have remarkably sculpted surfaces, but the 

 eggs of the majority of Crustacea are unexceptional, being rounded 

 or slightly ovoid. In contrast, the sperms of crustaceans are spec- 

 tacular in their range of form and size. They may be simple and 

 rounded, as in the Euphausiacea and some Cladocera, or they may 

 have numerous processes radiating from a central body, as in some 

 Decapoda. Sometimes, as in Argulus, the sperms look like tadpoles, 

 with a rounded head and a long tail. In the ostracods the most 

 fantastic sperms in the whole animal kingdom are produced. They 

 have long heads and may be up to ten times the length of the male 

 which produced them; further they are longer than those of a 

 whale or an elephant! Lowndes (1935) has suggested that such 

 enormous sperms cannot be functional, and it is a remarkable fact 

 that a great many ostracods are parthenogenetic, producing eggs 

 which do not need to be fertilised. 



Sperms are frequently enclosed in a case, the spermatophore, 

 which the male places on the female, generally rather accurately, 

 near the opening of the female ducts. 



In many copepods the contents of the spermatophore are 

 extruded by the swelling of some of the sperms, which increase 

 greatly in size and force out the sperms which are actually used to 

 fertilise the eggs. 



The sperms of decapods may be contained in a continous tube 

 which is plastered in a coiled mass on to the female, or they may 

 be enclosed in small separate spermatophores, each of which has a 

 stalk by means of which it is attached to a continuous strip. Finally 

 each spermatophore may be quite simple and separated from the 

 others. These three types are typical of the three major groups of 

 the Decapoda, but there are odd exceptions. The first type is found 



