230 
BUI.LETIN OF THE BUREAU OF FISHERIES. 
hundred brushes upon a single leg, and each brush contains from 50 to 100 setae, the 
bundles themselves being gradually concentrated toward the tip. In other words, 
each limb is furnished about its apex with from 5,000 to 10,000 sensory hairs, each 
of which is supplied with at least one nerve element. With such sensitive feet the 
lobster can feel its way securely at every step, whether by night or by day, as well 
as test every object before handing it up to the mouth. 
THE CENTRAL NERVOUS SYSTEM. 
The nervous system, the coordinating and regulating mechanism of the body, 
is composed of a complex series of distinct but closely related nerve elements, and 
each element consists of a ganglion cell and one or more outgrowing processes, the 
principal of which in certain cases is termed the nerve fiber. Three kinds of nerve 
elements or neurons have been described, as follows; (i) Coordinating elements, which 
lie wholly within the central system, the probable function of which is to coordinate the 
action of its parts; (2) motor nerve elements, which consist of a ganglion cell in the 
central mass and of a fiber process which passes out to a muscle or gland; and (3) 
sensory elements, composed of specially modified cells of the outer layer of the skin and 
of sensory fibers which enter the ganglia of the nervous system proper. Certain nerve 
fibers which pass out to the skin or its immediate neighborhood end in close relation 
with sensory cells and serve to convey impulses from them to the centers, while others 
conduct motor impulses from the centers to the muscles or glands. The epidermic 
cells of the skin may be regarded as the simplest sensory cells, or as the direct ancestors 
of such, and all the specialized sense organs, such as the eye or statocyst, are essen- 
tially modified patches or pockets of the outer skin layer. 
The most primitive sense being that of touch, it is not surprising to find in an 
animal like the lobster that virtually every part of the skin is capable of receiving and 
distributing either tactile or chemical sense impressions. The proper sense organs, 
however deep their final position in the skin or tissues, come into close relation with 
the nerve fibers with which each is abundantly supplied. The sense organs are thus 
a primary means by which any form of energy to which they are able to respond starts 
a series of changes which are finally translated into what are known to us as sensations, 
feelings, and other mental states. 
The lobster has a nervous system of the relatively simple “ladder” or “chain” type 
characteristic of the higher invertebrates (pi. xxxiii), in which segmentation, begun 
at a lower level in the animal scale, is the dominant character of its structure and 
instinct the ruling method of its response. Its reflexes and instincts are very precise 
and very stable, but not necessarily invariable, and, as we shall see at a later page, 
the lobster even at the fourth stage is able to modify its actions in relation to experi- 
ence and to form habits, and thus is gifted with a certain degree of what is usually 
defined as intelligence in vertebrates. The uprights of the ladder are the long com- 
missures of the chain, the rungs the transverse commissures, while the paired ganglia 
for each somite lie at the junctions of these parts. In addition to this cord with the 
