THE CRUSTACEA 21 



cation. Others, again, are believed to have a poisonous function. 

 The greatly developed cement-glands of the Cirripedia are possibly 

 related to the same group of structures. 



Phospliorescent Organs. In this connection may be mentioned 

 the power of phosphorescence possessed by many Crustacea. In 

 some cases this is due to a luminous secretion produced by certain 

 of the dermal glands. In the Euphausiacea and certain Decapoda, 

 however, complex and remarkable light-producing organs are present, 

 which were formerly described as "accessory eyes." 



Reproductive System. In the great majority of Crustacea, as in 

 other Arthropoda, the sexes are separate. Apart from certain 

 isolated instances, possibly abnormal and probably non-functional, 

 among Branchiopoda and Amphipoda, the only exceptions are the 

 sessile Cirripedia and some parasitic Isopoda (Cymothoidae and 

 Cryptoniscina), where hermaphroditism is the rule. 1 Partheno- 

 genesis is frequent among the Branchiopoda and Ostracoda, often 

 in more or less definite cyclical alternation with sexual reproduc- 

 tion. Where the sexes are distinct a more or less marked sexual 

 dimorphism often exists. The male is frequently provided with 

 clasping-organs for holding the female, and these may be formed 

 by the modification of almost any of the appendages, often the 

 antennules or antennae, or some of the trunk-limbs, or even the 

 mandibular palp (some Ostracoda). In addition, some of the 

 appendages in the neighbourhood of the genital apertures may be 

 modified for the purpose of transferring the sperms to the female. 

 In the higher Decapoda the male is generally larger than the 

 female and has stronger chelae. In the other groups the male 

 is often the smaller, and in many parasitic Copepoda and Isopoda 

 this disparity in size is carried to an extreme degree, and the minute 

 male is attached, like a parasite, to the enormously larger female. 

 The remarkable and complex sexual relationships of the Cirripedia 

 will be discussed in the section dealing with that group. 



The gonads of the Crustacea, as of other Arthropoda, are hollow 

 organs, the cavity communicating with the efferent ducts. They 

 are primitively paired but often coalesce partially or completely 

 with each other on the dorsal side of the alimentary canal. The 

 ducts are present only as a single pair, except in certain parasitic 

 Isopoda (Hemioniscidae, Liriopsidae), where two pairs of oviducts 

 are found. Various accessory structures may be developed in 

 connection with the efferent ducts in both sexes. The oviducts 

 may have diverticula serving as receptacula seminis (in cases where 

 internal impregnation takes place), and may be provided with 

 glands secreting envelopes or shells around the eggs. Similarly 

 the glandular walls of the vasa deferentia may secrete spermatophores 



1 According to recent observations (as yet unpublished) by Mr. Alf Wollebaek, 

 certain deep-sea Decapoda also are normally hermaphrodite. 



