68 A BIOLOGY OF CRUSTACEA 



sexual characters of the male. If the gland is removed the male 

 hecomes more like the female in general anpearance. Tf an ovary is 

 then implanted into such a male the similarity to a female is more 

 emphasised, and the specimen may develop a hrood pouch. 



Secondary differences between the sexes take a variety of forms. 

 The antennule of a male Dafihnia is long and movable, while in 

 the female it is short and fixed (fig. 32). Some male crabs have 

 enormous pincers (fig. 42), while the females remain a more normal 

 shape. The most striking differences are found in parasitic copepods 

 and isopods (chapter 8). The male may be only a minute fraction of 

 the size of the female, and quite different in shape. 



The sexes are not always separate; most barnacles are hermaphro- 

 dites, but some species have an extra complication. As well as 

 having bisexual individuals they have additional small males which 

 are only a fraction of the size of the hermaphrodites. These com- 

 plementary males attach themselves inside the mantle cavities of 

 the larger individuals and fertilise the eggs. The possession of male 

 organs by the larger individuals is thought to be an insurance 

 against the failure of the small, short-lived males, which probably 

 cannot feed and have to rely on food stores built up during the 

 free-swimming larval stages. 



Some of the Notostraca are also hermaphrodites, and sometimes 

 show an interesting geographical variation in the separation of the 

 sexes. Triops cancriformis has both males and females in North 

 Africa, but in Northern Europe only hermaphrodites are found. 

 In these the ovaries have scattered sperm-producing lobes among 

 the developing eggs. The cephalocaridan. Hatchinsoniella, is also 

 an hermaphrodite, but its ovaries and testes are quite separate. 



A change of sex during the life of an individual is a regular 

 feature in some shrimps. In Pandalus montagui some individuals 

 are primarily females, but others begin their lives as males then 

 develop female characteristics after about 13 months, and eggs 

 begin to develop in the reproductive organs. Isopods of the genus 

 Rhyscotoides show a similar chanee. but there seems to be an 

 absence of primary females, all individuals goine through a male 

 phase, and it seems probable that some individuals can change 

 back to male after a female phase. The genus is also peculiar in 

 that the female phase retains the male shape of the pleopods. 



The normal process of sexual reproduction involves the fusion of 

 a sperm with an e^. The Crustacea offer some exceptions to the 

 general rule, because certain species produce eggs which do not 



