GEV STAGE A— SEXUAL ORGANS 371 



There is no doubt that the sexual organs in all Crustacea were 

 originally paired. Some of them may, h6\vever, become unpaired, 

 either (as in most Copepoda and Thoracostraca) by the two germ glands 

 becoming connected by an unpaired uniting portion, or by the two 

 ducts uniting over a greater or smaller extent to form a common un- 

 paired oviduct or vas deferens, or by the ducts emerging through a 

 common aperture. We can, however, always recognise the double 

 nature of the sexual apparatus in some one (generally the larger) 

 portion of it. 



The ovaries and testes are either simple or branched or coiled 

 tubes or sacs which occupy in the body a dorsal position on each side 

 of the intestine, often between the heart and the intestine. They lie 

 in the trunk sometimes more to the front sometimes more to the back, 

 and sometimes along nearly its whole length. Where there are con- 

 necting portions between the germ glands of the two sides they lie 

 dorsally above the intestine. 



The genital apertures are found on the ventral side except in the 

 Gladocera and some Copepoda, where they lie dorsally. 



In the Entomostraca, setting aside the Cirripedia, the apertures lie, 

 as a rule, immediately behind the limb-bearing anterior division of the 

 trunk, at the limit between this and the limbless terminal division 

 called the abdomen. The single or double segment in which they emerge 

 is called the genital segment. There is thus in the Entomostraca no 

 definite constant segTaent of the body in which the genital apertures lie. 



In the Malacostraca, on the contrary, apparently including the 

 Leptostraca, the position of the genital apertures is definite and constant. 

 The male genital apertures everywhere lie in the most posterior 

 (i.e. the 8tli) thoracic segment, usually (Thoracostraca) in the basal joint 

 of the 8th pair of thoracic limbs. Tlie female apertures are in the 

 third from the last (the 6th thoracic segment, if we reckon in those 

 fused with the head), and mostly in the basal joint of the protopodite. 

 There are no exceptions to this rule. 



The Spepmatozoa of the Crustacea are often distinguished by their 

 remarkable size and shape. In the Becapoda they are provided with 

 radially arranged processes, and are, as also in other divisions of the 

 Crustacea, immobile. 



Numerous spermatozoa are often enclosed in a common envelope 

 (spermatophore) formed by the secretions of the glandular portion of 

 the male ducts. The egg's of many Crustacea possess besides a yolk 

 membrane other accessory envelopes secreted by the female ducts. 

 On adaptations for the care of the brood, see p. 379. 



Should the view prove correct, that the oviducts and vasa 

 deferentia and also the antennal and shell glands correspond with the 

 Annulatan nephridia, then, considering the different position of the male 

 and female genital apertures, several pairs (4 at the least) of the 

 segmental nephridia of the Anmdata have been retained in the 

 Crustacea. 



