STRUCTURE OF THE ISOPODA 317 
elsewhere almost of the same size as these. But the ex- 
ceptions to this statement are exceedingly numerous. 
Occasionally the first pair are opercular ; often they have 
the form of gnathopcds, that is to say, they are prebensile 
instead of ambulatory. Sometimes they are weak and 
slender, and agree in character with the three following 
pairs, at others they agree with only the next two pairs. 
Rarely the last three pairs are adapted for swimming in- 
stead of walking. The comparative lengths of the joints 
are also subject to much variation, although the second 
joint is scarcely ever shorter than the first or the third, the 
seventh pair of legs in Phreatoicus typicus, Chilton, and 
the first pair of the male in one or more species of Munna, 
offering perhaps rare exceptions. Marsupial plates are de- 
veloped in the females in varying number. The pedun- 
cles of the pleopods often carry coupling spines. The two 
first pairs of the pleopods are often modified in the male 
as sexual organs. Usually the inner branch in some of 
the pairs is branchial. All the pairs are sometimes want- 
ing. ‘The uropods vary greatly both as to shape, function, 
and their position on the sixth segment. The liver is said 
to consist of four or six backward-directed tubes. The 
heart is generally but not always elongate, and usually in 
part situated in the pleon. The ovaries and testes are with- 
out a median section. The young usually quit the brood- 
pouch with the last peraeopods still undeveloped. 
While in the Cumacea the pleon is in general very 
considerably narrower than the trunk, in the Isopoda this 
is only rarely the case, and sometimes the pleon even ex- 
ceeds the trunk in breadth. 
Definiticn of the Sub-order Isopoda. 
The cephalon or carapace with rare exceptions leaves 
free seven segments of the trunk. There are no branchial 
sacs in connection with the first maxillipeds, although these 
occasionally assist the respiration. Usually some of the 
pleopods have a branchial character. None of the lasi five 
pairs of trunk-limbs have natatory exopods. The first 
