85 
1916-17.] Experiments and Observations on Crustacea. 
alone among isopods are there six separate segments in the pleon). In 
this connection I might mention the comparison already drawn by Boas 
(1883, p. 565, footnote 2) between the “tail ” region of natant and reptant 
decapods and that of natant and reptant vertebrates respectively. 
General Discussion. 
It will have been noted that the mode of treatment in this paper 
differs from that usually followed in a present-day morphological disserta- 
tion. The difference lies in the emphasis laid upon functional con- 
siderations (one is apt to forget that physiological evolution invariably 
accompanies morphological). The original object was to obtain light on 
the functional heredity of the limbs of Ligia. In seeking an answer to 
one question provisional answers to others have been obtained. It will 
help to focus matters if I first separate the different issues. 
Morphological . — The ready method, due to Boas (1883), of discriminat- 
ing between Peracarida and Eucarida by means of the flexures at the 
mero-carpal and carpo-propodal articulations proves to be merely part 
(perhaps an unessential one) of a wider principle which involves the limb 
as a whole. In the Peracarida the limb tends to flex alternately in one 
and the same (principal) plane. The arrangement is usually tri- alternate, 
but may become quadri-alternate, as in some of the Arcturidse. In the 
reptant Eucarida the flexion-complex is of another kind, more difficult to 
describe in few words. Apparently correlated with the existence of a 
tri-alternate flexion-complex is a separation of the walking limbs into two 
opposed groups. According to the direction in which the proximal part 
of the limbs project from the body we distinguish three types of limb- 
taxis : one widely prevalent, the other two more restricted in occurrence. 
These results are derived by following the existing classification, and 
to this extent merely give added colour to what is already known.* Can 
they be put to further use ? Here I shall deal only with the Isopoda, and 
from want of the requisite knowledge in a very tentative way. 
The Asellota form a homogeneous group, which has been revised of 
late years by Hansen (1905). The application of our principle to this 
sub-order might therefore be taken as a test case for the purpose of 
assessing practical utility. Hansen divides the Asellota into Asellidse, 
Stenetriidse, and Parasellidse. Now, Asellus has isopodan limb-taxis; 
* The fact that the question of limb arrangement has never been systematised partly 
accounts for the difficulty experienced in obtaining illustrations of inaccessible animals 
from aspects that make plain the taxis. 
