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PACIFIC SCIENCE, VoL XIII, April, 1959 
of the pleopods to respiration in the Isopoda 
and the terrestrial Amphipoda. 
In the Isopoda, the pleopods show a series 
of modifications for respiratory purposes. In 
marine species, the pleopods are used both in 
swimming and for respiration. In the terres- 
trial species, as in the amphipods, they are no 
longer of use in locomotion. In the primitive 
state, both exopodite and endopodite in all 
pleopods are respiratory. In the Ligiidae, the 
exopodite is the primary respiratory organ 
and the endopodite is a secondary one. In the 
more primitive terrestrial species, such as in 
the Oniscidae, the endopodites are trans- 
formed into branchial lamellae and take over 
most of the respiratory function. In Porcellio 
and other genera, the exopodites have com- 
pletely lost their respiratory function and act 
as lamellae protecting the endopodites from 
dessication. However, in the most advanced 
terrestrial species, the endopodites again re- 
gress and are replaced by pseudotracheae 
("white bodies” or "tracheal glands”) on the 
inner side of the exopodites. It is these pseu- 
dotracheae which enable the desert-dwelling 
species to survive. The pseudotracheae have 
evolved in three independent lines compara- 
ble to and surpassing the development of a 
terrestrial mode of life in the amphipod 
genera, Orchestia, Talorchestia, and Talitrus. 
In marine species of amphipods, the ple- 
opods have three functions (Watkins, 1939, 
1940). They are used for swimming, they 
draw a constant current of water over the 
gills, and the same current, filtered by the 
gnathopods, serves as a food vehicle. 
With the development of extensive pleon 
muscles and the change to a supralittoral 
jumping habit, pleopods are no longer used 
in feeding. Food is obtained actively and the 
gnathopods are used to grasp and convey it 
to the mouth. However, all supralittoral spe- 
cies possess fully developed pleopods, sug- 
gesting that they are still used in respiration. 
Since supralittoral species have not lost the 
power to swim and occasionally need to swim 
if they are to survive, their pleopods may 
fulfil two of these functions. 
Among terrestrial species there is a trend 
towards reduction or complete loss of pleo- 
pods except for vestigial triangular stumps, 
but there are exceptions to this. New Zealand 
species from leafmould collected 2,000 to 
3,000 feet above sea level have fully developed 
pleopods, e.g., Orchestia sinbadensis Hurley 
(1957). 
A completely terrestrial habitat removes 
any need of pleopods for swimming. Accord- 
ing to Lawrence (1953), Talitrus eastwoodae 
"never swims when placed in water and is 
evidently unable to raise itself above the bot- 
tom of a glass jar, moving round in it pre- 
cisely the same way as if walking on dry 
land.” The absence of pleopods in O.patersoni 
(Stephensen, 1938) implies that sufficient oxy- 
gen is available to the respiratory surfaces 
without need for a constant current of air 
created by the pleopods. As might be ex- 
pected, there is in most species a considerable 
development of respiratory surfaces corre- 
lated with this loss. "The gills of the anterior 
pair of pereiopods are rather large, in the fe- 
male especially so in pereiopod 2” (Stephen- 
sen, 1935) . Talitrus sylvaticus is a good example 
of this marked development; it has the gills of 
the fourth peraeopod, as well as those of the 
second, much enlarged and elongated (Hur- 
ley, 1955). Possibly, the retention of pleopods 
in some terrestrial species depends on a bur- 
rowing habit, in which case the available air 
in the burrow might stagnate more rapidly 
were not the pleopods used to increase the 
circulation. A correlation has also been sug- 
gested between burrowing and reduction of 
eyes, or even loss of pigment, as in Orchestia 
marquesana (Stephensen, 1935), but evidence 
of this is very limited. 
OTHER MODIFICATIONS 
In the Isopoda, at the same time as the de- 
velopments in the pleopods described above, 
other tendencies have appeared. The antennae 
have become reduced, probably with an as- 
