238 Proceedings of the Royal Society of Edinburgh. [Sess. 
flaps of the tail cause the limb to break off at its base, and the lobster 
escapes. 
(2) If a sharp cut be made across a walking-leg and the animal at 
once released, the leg becomes extended for a few seconds, and the lobster 
shoots backwards by flaps of its tail. After a definite time (usually about 
fifteen seconds), the stump is flexed inwards to the middle line and violently 
pulled forwards by the chelate walking-legs and the maxillipeds. In a 
few cases removal of the remaining part of the leg at the second joint 
takes place, but as a rule the cut end heals up in a few days and the 
part is retained. If a lobster which has not autotomously removed a stump 
be kept over the moulting period, that leg does not regenerate, while those 
which have been thrown off at the base are replaced. 
In order to study further the points revealed by the above observations, 
the structure of the basal segments of the walking-leg must be examined. 
The second segment (fig. 4, (2)) consists of a hollow triangular pyramid 
hinged at two of its basal points on to the first segment. Yentrally a 
flexor muscle has its tendon attached to the lower border, while dorsally 
an extensor from the rounded edge (which represents two sides of the 
pyramid base) passes into the cavity of the first segment (fig. 5, e). The 
inner side of the dorsal wall of the second segment gives attachment to 
a muscle of which the tendon is inserted on the ventral edge of the third 
segment. This muscle (fig. 6, a) by its position may be regarded as a 
flexor ; but since movement at the joint between the second and third 
segments is very limited, it probably does ilot function as such. 
The third segment articulates with the second by a hinge joint, and at 
the point of attachment of the muscle described above (figs. 4 and 5, /) 
a furrow begins at its lower edge and passes round the greater part of 
the lower edo^e of the segment at a distance of about one millimetre from 
the edge. 
After autotomy at the second joint, a ring of calcareous material 
corresponding to the part of segment three below the groove, / is found 
attached to the stump (fig. 9, h). The muscle a is contracted, and thus 
the ring is pulled into the cavity of the second segment. The division of 
the limb has thus taken place along the groove at the base of the third 
segment, and, where that groove ended, along the arthrodial membrane 
joining segments two and three (figs. 7 and 8). 
The factors controlling autotomy of the walking-leg were further 
investigated, with the following results : — 
(I) Division of the extensor muscle of the second segment prevents 
autotomy occurring. 
