84 
Proceedings of the Royal Society of Edinburgh. [Sess. 
“(5) The pleon is formed of six separate segments, and is better 
developed than in most Isopoda.” 
Even in the absence of any complete record as to the mode of use of 
the limbs the question is worth elaborating for a reason that will become 
more plain in the next succeeding section. We shall take the points under 
their appropriate numbers as Chilton has set them down. 
(1) Greater narrowness of pleon than of peraeon is almost a mechanical 
necessity of the peculiar mode of gait present in Gammaridea. As regards 
lateral compression of the peraeon itself, Chilton in his discussion mentions 
amphipodan genera in which the body is more or less cylindrical, viz. 
Caprella, etc., Gorophium, Haplocheira ; and others in which the body is 
as much flattened as in most Isopoda, viz. Icilius , Iphigenia, and Cyamus . 
Many of these genera do not possess amphipodan taxis. 
(2) There is no particular reason to suppose, as Chilton suggests, that 
downward prolongation of the pleura should arise “ quite independently 
of the similar adaptation in the Amphipoda.” Along with the gam- 
maridean gait goes a peculiar use of the abdominal segments, which are 
rapidly flexed and extended. As he suggests, these pleural plates may 
have a function of protection for the pleopods ; this would come into play 
particularly in extension of the abdomen. Moreover, beautifully fitted 
imbricating pleural plates may give mechanical support and guidance to 
the body segments in ventri- and dorsi-flexion (compare the arrangement 
in the peraeon of an amphipod with that in the peraeon of an isopod). 
(3) Here I have no comment to make. 
(4) Examination of Chilton’s plates shows that the posterior three 
thoracic limbs have sufficient stretch to be thrown well dorsalwards. As 
he himself states, these limbs are “ more amphipodan and flattened.” The 
modification of the uropoda might well be correlated with what was said 
under (2) immediately above ; in progression the uropods of Gammarus 
brush against the walking-surface. 
(5) See also under (2) immediately above. 
Should it be found on further investigation that Phreatoicus does 
employ the amphipodan mode of gait (if only to the extent of jerking 
itself forward by extension of the abdomen), we should of course be in 
a position to reverse the argument and to formulate clear correlations 
between structure and function. We should understand better the raison 
d'etre of the so-called lateral compression of Amphipoda, or rather of 
vertical pleural plates, something of the principles governing build of 
uropods, and, lastly, the conditions that permit of concrescence of abdominal 
segments in Isopoda as compared with Amphipoda (in Phreatoicus almost 
