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VI.—Experiments and Observations on Crustacea: Part III. Limb-Flexures and Limb-Taxis in the Peracarida
Published online by Cambridge University Press: 15 September 2014
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 correlations 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 distinguished from analogy. The systematic study of analogies would appear to be more worthy of consideration than is generally acknowledged.
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References
LITERATURE
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