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is the same as in the Decapoda, but the segments correspond- 

 ing to the posterior seven are distinct from one another. 

 The first ambulatory appendage thus corresponds to the 

 second maxilliped of the Decapoda. Of the six appendages 

 of the abdomen the three first are natatory and similar to 

 those of the lobster or shrimp, while the hinder three are 

 short, stiff, and directed backwards ; these function as spring- 

 ing or jumping limbs, but the force of the jump is due to the 

 bending and straightening of the abdomen itself. 



The apertures of the male generative organs are at the 

 base of the last pair of thoracic legs, those of the female at 

 the base of the last pair but two, in both cases in the same 

 relative positions as in the Decapoda. Special intromittent 

 organs are wanting, and the exact mode in which fertilisation 

 is effected is at present unknown to me. But the male is 

 generally nearly twice as large as the female, and carries her 

 bodily about between his legs with great persistence. Any 

 one who turns over fronds of sea-weed or stones on the shore 

 at low tide will see couples of the common Gammarus thus 

 united. The active part of the male and the passive behaviour 

 of the female in this stage of the process of reproduction 

 corresponds to the difference in their dimensions and strength, 

 and is a sufficient explanation of that difference. 



In addition to the general superiority in size the males 

 possess other unisexual characters. These usually consist in 

 a greater development of the grasping organs on the anterior 

 thoracic legs. The differences between the sexes are well 

 exemplified in the species Podocerus falcatus (Mont.). This 

 species builds tubes for itself out of mud on the surface of 

 logs or other objects in the water. I have found it extremely 

 abundant at Falmouth on floating timber just at the level of 

 the surface of the water. The secondary sexual characters 

 are so conspicuous that the different forms have been de- 

 scribed as three distinct species, namely, the mature male as 



