77 
1916-17.] Experiments and Observations on Crustacea. 
sub-heading as associated with lateral projection of basipodites from the 
body. Just as in the amphipodan type, the limbs with uniplanar tri- 
alternate flexion-complex show a separation into two groups opposed 
in the direction of the chief articulations. The anterior basipodites 
project laterally and backwards, the posterior laterally and forwards. 
The consequence is that the dactylopodites still dig into the ground 
with a backward or forward inclination, as the case may be. 
This type is present apparently in all the Tanaidacea ; in the atypical 
Flabellifera (Gnathiidse, Anthuridse); perhaps in some Valvifera (Arc- 
turidse) and Asellota; and in the less typical Amphipoda (Caprellidse, 
Cyamidse). It is therefore widely prevalent, whereas isopodan taxis is 
wholly, and amphipodan almost wholly, confined to the respective orders 
indicated by the adjectival prefix. 
A B C 
Fig. 3. — Diagram to show the direction of the basipodites in different forms of limb-taxis. 
A, tanaidacean ; B, amphipodan ; C, isopodan. 
In the diagram in Fig. 3 the three forms of taxis are schematically 
represented. 
The Primitive Taxis . — We come now to an important question from 
a systematic point of view. Of the three types of limb-taxis, which is 
the most primitive ? 
The isopodan may probably be set aside at once as the least frequent 
and highly specialised. In fully developed form it demands great 
separation of the basal parts of the limbs and consequent breadth of 
body. It seems to be unique in the animal series. 
The next in increasing order of frequency is the amphipodan, 
probably also specialised. A deep and narrow figure, at least in an 
aquatic animal, is mechanically ill - adapted for prone progression ; 
without lateral grip on the walking surface it can readily be washed 
over on its side. 
The mechanically simplest form, the earliest to occur (from Mysidacea 
onwards), the most widely prevalent, and for all these reasons the most 
generalised , is the tanaidacean, in which the limbs spread more or less 
