NATURAL HISTORY OF AMERICAN LOBSTER. 
341 
Perhaps the most interesting morphological change which appears at the fourth 
stage, though by no means the most striking, is the torsion of the great chelipeds, described 
in chapter vii. The differentiation of the big claws, which come in time to equal one 
half the weight of the entire animal, is preceded by a permanent twist which has chiefly 
affected the fifth segment. While the lobster in the fourth stage is limber in every joint, 
the fusion of the second and third podomeres occurs shortly after this molt. 
Lobsters after the larval period, and preeminently in the fourth and fifth stages, often 
exhibit the phenomenon known as “feigning death.” When stroked with any object 
or when water is squirted on them with a pipette they will roll over and straighten out 
as if paralyzed. Their appearance when in this state is very different, however, from 
that of a dead animal. The phenomenon appears to be a somewhat sporadic reflex 
response, but it is interesting to find it appearing for the first time when the animal is 
about prepared to sink to the bottom, and to assume more fully the habits of an adult 
animal. (See 149, p. 184.) 
Fourth-stage lobsters when approaching the end of their period frequently go to 
the bottom in shallow aquaria, hide under stones or any accessible objects, and even 
burrow in mud or sand. 
The instinct of fear also appears in this stage and for the first time, associated with 
the hiding and burrowing tendencies. These are possibly evoked by the development 
of that contact-irritability which, as Hadley remarks, seems to come suddenly into 
play toward the close of this period. Burrowing is a kind of behavior in which the 
lobster frequently indulges from this time onward throughout life. The burrows serve 
a fourfold purpose — for concealment and therefore for protection, as a point of vantage 
from which to watch and seize their prey, and probably as a means of avoiding strong 
light, especially when adult, and particularly when confined in relatively shallow “parks” 
or pounds. 
Digging the hole is an instinctive act; but returning to the same burrow of holding 
to the same crevice for the purpose of defense, for hiding, or for seizing the prey, so 
marked in all the later stages of both young and adults, is a distinct mark of intelli- 
gence, a habit of returning to the same spot being formed through association. 
An interesting phase in the behavior of the fourth-stage lobster, as described by 
Hadley, is its rheotactic response or tendency to head into the current, which, with 
its other reactions, will be later discussed. 
Color in the fourth stage . — At this period the range of color variation is much greater 
than at any previous stage, but color change no longer follows so promptly change in 
temperature, in the illumination, or in the intensity of other effective stimuli. The 
chromatophores or pigment cells of the skin have so multiplied as to form a continuous 
screen to the parts below. The former transparency of the larva is thus reduced in 
the same degree that the depth and brilliancy of its colors are enhanced. 
The exoskeleton is now reenforced for the first time with considerable deposits of 
mineral salts, especially of lime. It is still quite translucent, but of a delicate light- 
blue tint, as appears at the molt. The body of the lobster, and the cephalo-thorax in 
particular, is studded with sensory hairs. The hair pores constantly increase in number 
