76 Proceedings of the Koyal Society of Edinburgh. [Sess. 
At this juncture I shall quote from Caiman (1909) a passage that 
caused me perplexity. “ The lateral compression of the body in most 
Amphipoda has led to a separation of the thoracic legs into an anterior 
group of four (the two gnathopods and the first two perseopods) and a 
posterior of three, which are opposed to each other in the direction of the 
principal articulations.” Then follows a sentence or two describing the 
structure of these limbs in more detail, to which is subjoined this footnote : 
“ The correlation between the lateral compression of the body and this 
grouping of the legs is well illustrated by comparison with Phreatoicus , 
the only isopod where the body is laterally compressed and where the 
legs are divided into two groups exactly as in Amphipoda.” 
One would be bold perhaps to dispute the existence of any correlation. 
Independently of this, it is difficult to see how the lateral compression has 
led to a separation of the legs into groups. Here a more detailed 
specification seems to be necessary. We meet with separation of the 
legs into two groups in cases even of isopodan limb-taxis (Yalvifera, 
Flabellifera), which involves breadth of body. Apart from either of these 
two forms of limb-taxis, we meet with separation of the legs into two 
groups showing contrast in the direction of the principal articulations, 
in other Amphipoda and Isopoda and even in Tanaidacea that show no 
particular lateral compression of the body ; in all these the basipodites 
have a still different orientation, neither flexed under the body (isopodan 
type) nor in an antero-posterior direction (amphipodan type), but projecting 
laterally outwards. The real correlation, as Caiman indicates in his 
footnote (as if still beset with qualms as to his logic), hinges upon the 
peculiarly antero-posterior plane of flexure of the limbs, not upon their 
separation into groups. Even in the end the “ lateral compression of the 
body ” is more apparent than real, being largely if not wholly due to the 
vertical depth of the coxal plates.* Chilton (1894), too, describing the 
genus Phreatoicus , remarks : “ The lateral compression of the body is not 
so great and is chiefly seen in the pleon, where the pleura of the segments 
are produced downwards.” 
The matter therefore reduces itself to this : When the taxis is exactly 
antero-posterior, the coxal plates, if present, are antero-posterior. As we 
shall immediately see, the separation of the legs into groups is an 
independent modification. 
Tomaidacean Limb-Taxis . — As tanaidacean we may for convenience 
designate the type of limb- taxis incidentally referred to under the last 
* “The large coxal plates on the thoracic somites projecting downwards increase the 
depth of the body and add to the appearance of lateral compression” — Caiman (1909). 
