62 Proceedings of the Royal Society of Edinburgh. [Sess. 
always adheres to the head-covering, by this last means the exuviation of 
the headpiece is incidentally aided. 
Unlike the posterior covering of the animal, which is left behind in a 
fairly coherent or intact condition, the front integument may be torn apart 
into its constituent limbs, into a single portion representing the middle 
dorsal covering of segments 2, 3, and 4, and into a headpiece to which the 
antennae and first thoracic somite with its pair of walking legs adhere.* 
As a rule, the last two larger poi Jons (dorsal covering of segments 2, 3, 
and 4, and composite headpiece) come away united. Where separation 
occurs between them, the animal may, for a day or two after moult is other- 
wise complete, continue to wear on its back the three middle dorsal portions 
of somites 2, 3, and 4. A somewhat indistinctly recorded observation of 
Herold (p. 476, fourth paragraph) may refer to the occurrence of a similar 
phenomenon in Oniscus. 
The limbs in any newly moulted half of the body being for a short 
period incapable of function, we find that, just subsequent to posterior 
moult, Ligia walks by means of its anterior four pairs of thoracic limbs, 
the hinder part of the body and uropods trailing limply behind ; while, 
just subsequent to anterior moult, it walks on the hinder three pairs of 
limbs. In the latter case, should the fifth pair be thrown forwards simul- 
taneously (as a rule these limbs move alternately), the centre of gravity of 
the animal now lies behind the anterior line of support ; in consequence, 
the front of the body is tilted upwards to an angle of about 40 degrees with 
the horizontal plane. So soon as ever these limbs are drawn backwards 
again, the head-end of the ahimal drops to the ground. In progression by 
simultaneous forward and backward movement of the fifth pair of limbs, 
the fore-end of the body executes a series of upward-rising and downward- 
flopping movements extraordinary to witness. Should the animal come to 
rest with the fifth pair of limbs thrown forwards, its curiously fixed posture 
might remind one of Owen’s restoration of Mylodon ! 
In this connection I shall quote Friedrich’s account of Porcellio, which 
I lighted upon only after the above paragraph was written. “ The move- 
ments of the animals at this time are very awkward. On moult of 'the 
hinder part of the body . . . they use the front four pairs for progression. 
On moult of the front half of the body, the three new pairs of the fifth, 
sixth, and seventh thoracic segments (of which the first pair is directed 
* Macerated unmoulted Ligise may fall apart by disruption at precisely the same sites. 
Such animals are probably near the moult, and the fact is of physiological interest. It may 
also be of practical interest if moulting of Peracarida and of other malacostracan groups is 
to be systematically investigated (see next section). 
