93 
1 916 - 17 .] Experiments and Observations on Crustacea. 
Summary. 
1. Each walking leg of Ligia shows three prominent flexures arranged 
alternately in one plane. Generally speaking, the planes of flexure of the 
whole series of seven limbs are transverse to the body, the basipodites 
flexing medially ; this arrangement, here called isopodan limb-taxis, is 
associated with clinging power. 
2. In Amphipoda and in Tanaidacea there are likewise three chief 
flexures in the limb, which are also arranged tri-alternately and in one 
plane. In the Tanaidacea the basipodites project laterally not medially. 
In the Amphipoda the basipodites flex antero-posteriorly. Tanaidacean 
limb-taxis seems to be the primitive form. 
3. The tri-alternate flexion-complex in each limb of these Peracarida, 
as well as in certain limbs of Cumacea, is a reptant feature ; so is the 
separation of the seven limbs into two opposing groups : an analogy with 
the limbs of reptant vertebrates is here traced. By simple inspection it is 
usually possible to say if a crustacean limb used for swimming has been 
secondarily modified from a reptant limb. 
4. The flexion-complex in the limbs of reptant Eucarida is neither 
tri-alternate nor uniplanar as in the limbs of reptant Peracarida. This 
distinction is of more importance than that suggested by Boas, which 
relies upon the amount of flexion possible at the mero-carpal and at the 
carpo-propodal articulation respectively. 
5. The Peracarida, like the Decapoda, may be divided into natant and 
reptant forms, with intermediate links. Of the reptant forms only those 
with isopodan or amphipodan limb-taxis have shown themselves capable 
of adopting a land life. 
6. It is suggested that Phreatoicus may in progression employ its 
posterior peraeopods or its pleon in the same way as a Gammarus. Should 
it be found that the animal does so, one could formulate additional cor- 
relations between structure and function ; these would involve vertical 
pleural plates, uropods, and the number of free segments in the pleon. 
7. Considerations relating to limb flexures and limb-taxis have been 
tentatively applied to the classification of the Isopoda. 
8. The different forms of limb-taxis in the Peracarida and certain 
features involving the flexion-complex, present problems analogous to the 
rotation and torsion that occur in the fore-limb of a developing mammal. 
9. In a discussion on homology and analogy it is pointed out that 
homology in the sense of homoplasy (Lankester) cannot be effectively dis- 
tinguished from analogy. The systematic study of analogies would appear 
to be more worthy of consideration than is generally acknowledged. 
