7 5 
1916-17.] Experiments and Observations on Crustacea. 
rather than as organs of locomotion. These facts possibly convey a hint 
that the well-developed locomotor power of the corresponding limbs of 
Oniscoidea represents reacquisition of a function temporarily in abeyance 
in their more immediate marine ancestors. 
The general type of limb- taxis described as present in Ligia I shall 
henceforth call “isopodan.” By this term is implied merely that the limbs 
with tri-alternate flexion-complex lie on the whole in planes transverse to 
the body, the basipodites being directed medially. Isopodan limb-taxis 
may be subdivided into varieties. For example, the “ oniscoidean ” variety 
is peculiar in that the various planes of flexion of the limbs vary 
uniformly in direction as one proceeds from front to back, not abruptly as 
in many Flabellifera and Yalvifera. 
Am'phipodan Limb-Taxis . — In amphipods we meet with a slightly 
different type of limb-taxis. In each thoracic limb one can detect 
a uniplanar tri-alternate flexion-complex, composed of flexural units 
containing elements identical with those in the isopod limb. In most 
amphipods the first four limbs have an ischio-flexure, the apex of which 
points backwards and which is analogous to a mammalian elbow, while the 
ischio-flexure in the posterior three limbs points straight forwards like a 
knee. The front four dactylopodites might be considered as hooking into 
the ground so as to resist a backward pull on the part of the limb, the 
last three dactylopodites (at least in the conventional position often 
adopted in illustrations) then digging into the ground in the opposite 
direction. (Deviations from type and other complications will be discussed 
later.) To the amphipodan taxis the simile of a boat held fast by a series 
of boat-hooks projecting all round the gunwale does not apply ; the taxis 
is exactly antero-posterior and oppositely directed, and might be roughly 
compared with that seen in the limbs of a horse. An identical limb-taxis 
occurs in the isopod Phreatoicus. 
The basipodites in this case undergo flexion in a plane parallel to the 
sagittal. The coxopodites, virtually incapable of flexion at the coxo-somitic 
joint, in typical amphipods take the form of great, laterally compressed 
segments (we neglect the extra expansions on the first four), which, 
possibly for reasons connected with muscular efficiency,* lie in the same 
plane as that in which the basipodite flexes. 
* On similar considerations one would expect a (mean) vertical transverse flattening of 
coxopodites in Ligia. It might be urged, however, that such a disposition would by 
resistance interfere with forward progression of the animal in the fluid-surrounding 
medium. Perhaps more important is the fact that in the typical isopodan limb with fused 
coxal plate the articulation between coxopodite and basipodite is analogous to a ball-and- 
socket joint, the coxo-basal articulation in the amphipod being a simple hinge. 
