78 Proceedings of the Royal Society of Edinburgh. [Sess. 
directly outwards, affording a wide basis of support and attachment. 
Through the kindness of Dr J. H. Ashworth I had an opportunity of 
examining not only Mysis but fine spirit specimens of the syncaridan 
Anaspides. In this latter animal, too, the basal parts of the limbs 
flex outwards. The proof from comparative morphology seems to be 
complete.* 
Conditions attaching to Reptant Limbs. 
We shall now return to a discussion of the flexion-complex and of 
the grouping of the limbs into two opposed sets. 
The Tri-alternate Flexion-Complex . — The quadri-alternate and tri- 
alternate flexion-complex in the limb of land mammals and of Ligia 
respectively is undoubtedly due to a long heredity of walking or 
crawling habits. Among the Vertebrata we meet with alternate flexion 
within the compass of the limb for the first time in Amphibia. In 
fishes there is hardly a hint of such an arrangement ; in swimming 
mammals the complex shows all degrees of reversion to the flexural 
simplicity of a fin. 
If we take the Peracarida as a group comparable to the Vertebrata, 
we find little or no alternate flexion in the limbs of swimming Mysidacea ; 
in those of the Cumacea, especially in the posterior thoracic or fossorial 
limbs, we see indubitable evidence of a tri-alternate flexion-complex ; 
finally, in the Tanaidacea, the Isopoda, and the Amphipoda the walking 
limbs may be said to show marked tri-alternate flexion. 
The argument may be continued from the functional side. Imagine 
an unbending peg or projection (ventrally hinged and controlled by 
opposing antero-posterior muscles) to be used not for swimming but 
for progression over the ground. The limb pointing forwards, its distal 
end is fixed to the walking surface ; the proximal end then describes 
an arc of a circle round this fixed point. The body of the animal, as 
it moves forward in correspondence, first rises and then falls again. 
This, in itself, like the galloping of a horse, is a strenuous and energy- 
wasting mode of progression. The limb has now to be moved forward 
again ; it can do so only if there is a possibility of alternate flexion. 
(On analogous reasoning one might show an advantage in alternate 
flexion in a limb used for prehension.) 
The general deduction appears unassailable. As an example of its 
application to specific cases, we may take the swimming function sub- 
* One might perhaps associate the development of coxal plates (Isopoda and 
Amphipoda) with the more specialised limb-taxis. 
