NATURAL HISTORY OF AMERICAN LOBSTER. 
235 
The exact analysis of the sense organs of the higher Crustacea is still a vexed problem, 
and the literature of the subject far from satisfactory.® In the description to be given 
I shall follow in the main the account of Prentiss (217), who worked upon the common 
prawns, Palcemonetes and Crangon vulgaris, with which the lobster undoubtedly agrees 
in these particulars. The sensory bristles of decapods have been found to conform to 
two types: (x) The tactile, and (2) the olfactory, or better, the chemical setae which 
are sensitive to chemical stimuli. The former have straight, long, and often plume-like 
shafts, and at the base of each a spherical enlargement is formed, which, owing to its 
thin wall, permits the hair to swing freely as upon a joint. Bristles of this type occur 
all over the body and appendages, and the “auditory hairs’’ of what has been called 
the “ear-sac” or otocyst (fig. 2 and 4, pi. xxxv) are of this form. According to Prentiss, 
each is supplied with a single nerve element. The “olfactory” or “chemical” bristles are 
shorter, more cylindrical, or less tapering chitinous tubes, with no marked basal swelling. 
Their tips are either perforated or possess so thin a wall as to permit the ready diffusion 
of chemical substances from the water to the inside of the shaft. Each bristle is supplied 
with a cluster of nerve elements, which may be very numerous, their fibers ending free 
in the shaft, but not penetrating to its apex. Such setae are apparently more highly 
specialized and are restricted to the small antennae, where they are called olfactory 
hairs, or to the mouth parts, where they are often spoken of as gustatory bristles, though 
it is probable that their functions are the same wherever found. 
RELATION OF THE SETAJ TO HATCHING AND TO MOLTING. 
The way in which these sensory hairs are formed and renewed at each molt is very 
interesting. The subject has been investigated by a number of naturalists, but in the 
brief account which follows we shall depend mainly upon the observations of Prentiss. 
Each hair is secreted by a number of matrix cells which send their processes up into its 
shaft. In preparation for the molt the protoplasm recedes from the shaft of the hair 
and its matrix cells sink into the tissues and with other cells form a “papilla” around 
the nerve fiber and begin to secrete a new hair. This condition lasts for a long time in 
an adult animal, but for a few days only in the larva, which often passes several molts in 
the course of a week. The cuticle which is to form the new shell and hair is secreted 
under the old which is soon to be cast off, but the new hair is invaginated, so that below 
the level of the skin its wall is double, while its tip only projects into the hollow shaft of 
the old hair above it. The walls of the double hair tube are thus continuous with each 
other and with the general cuticle which is to form the new shell. 
In this condition the hairs may be compared to the fingers of a glove which have 
been pushed in or telescoped, so that their tips only project from the surface. When 
the lobster is ready to molt every new hair on its body is in this condition. Now at 
each molt we always find between the old and new cuticle a sticky, homogeneous 
substance which adheres both to the old shell and to the tips of the new hairs. Molting 
a For a review of this subject, see Bell: The reactions of crayfish to chemical stimuli. Journal Comparative Neurology and 
Psychology, vol. xvi, p. 299-326. 1906. 
