79 
1916-17.] Experiments and Observations on Crustacea. 
served by the posterior three thoracic limbs o£ the Munnopsidse. Apart 
from the mere morphological argument — cf. Hansen (1904) on the classi- 
fication of the Asellota, and Beddard’s (1886) figures of Munnopsidse in 
the “ Challenger ” Report — this is obviously a new adaptation of previously 
reptant limbs. According to Sars (1889), Desmosoma, Munnopsis, 
Ilyarachna, and Eurycope all employ these limbs for backward pro- 
pulsion. If it should be the case that the Mysidacea use the thoracic 
limbs only for forward swimming, and that backward swimming by 
means of these limbs is otherwise unknown among Peracarida, the 
phenomenon would, from a purely physiological point of view, be 
remarkable. Observations on the functional mechanism of the three 
posterior walking limbs of Ligia (as, for example, during anterior 
moult — see the immediately preceding paper of this series) would, how- 
ever, indicate a possibility of deriving the backward progression of 
Munnopsidse from a facility first acquired by reptant forms.* 
As already indicated, the tri-alternate flexion- complex is no character- 
istic of the decapod walking limb. The limbs of reptant forms of the 
Eucarida have developed along another line from those of reptant forms of 
the Peracarida. At the same time the general principle holds, viz. that 
an elaborate flexion-complex found in any limb used for swimming 
means that the natant function has been secondarily derived from a 
more primitive reptant one — cf. the last pair of thoracic limbs in the 
Portunidse. 
Separation of Limbs into Two Groups. — As possible causes of the 
distinction between the two sets of limbs two contingencies suggest them- 
selves. The phenomenon may be associated with the presence of the 
thoracic brood-pouch found throughout the group — one might suppose, 
for instance, that the basipodites converging from both ends of the thorax 
towards the middle might help to protect the brood-pouch. On the other 
hand, the separation might be connected simply with the function of 
locomotion. 
The fact that there is no such separation in the Mysidacea, which 
possess a brood-pouch, and the further fact that the separation becomes 
apparent for the first time in Cumacea, which are reptant, would indicate 
that the division follows upon the acquisition of a crawling habit. The 
comparative evidence is not limited to the Peracarida. In Geoffrey 
Smith’s (1908) figure of Anaspides in the natural position for walking 
(see also Cambridge Natural History , vol. iv, p. Ill), the separation is 
* Chilton (1894) mentions that the anthuridan Gruregens runs “backward and forward” 
with equal facility. 
