96 H. J. HANSEN. 
between the terminal abdominal margin and the uropods, 
with the result that the animals can live closely clinging to a 
firm body, and by movement of the pleopoda produce through 
that aperture a current of water to the rami adapted for 
respiration. 
Nesicopea abyssorum (Bedd.) has a round foramen on 
the end of a protuberance considerably above the posterior 
margin of abdomen, and this foramen is the upper part of an 
exceedingly deep incision or transformed notch, but the major 
distal and lower part of this incision looks like a suture in the 
mesial line to the lower margin of abdomen. ‘The result of 
this structure must be that this rather large animal can walk 
on very soft muddy bottom with the lower margin of abdomen 
touching the mud, but yet get pure water through that fora- 
men to the branchiz ; according to Beddard the two specimens 
known were taken ina depth of 1070 fathoms, and the bottom 
was ‘blue mud.” In a species of Dynamene from the 
Mediterranean I find about the same: a foramen on the end 
of a protuberance above the end of abdomen, but the distance 
between this end and the foramen is proportionately shorter 
than in Nesicopea. In some forms (Dynamenella, 
Cerceis, etc.) there is considerable difference in the shape 
of the notch in the two sexes, which suggests that some 
difference in the biology of the sexes may exist. Hemi- 
sphzeroma pulchrum and allspecies of Sphzeroma have 
no trace of a notch, and the posterior margin of abdomen is 
broadly rounded, but in these forms the three anterior pairs of 
thoracic legs are furnished with very long and stiff natatory 
setee not met with in any other genus; their habits are, there- 
fore, probably more natatory than those of other marine genera; 
they can easily get pure water to the branchiz from below, 
which agrees with the total absence both of notch and of 
groove on the lower distal part of abdomen. According to all 
these examples (Plakarthrium, Leptospheroma, Nesi- 
ccpea, Dynamenec, Hemispheroma, Spheroma) we 
must assume that the shape of the end of abdomen is an 
important feature, being developed in various ways according 
