230 BUlvIvETIN OF THE BUREAU OF FISHERIES. 



hundred brushes upon a single leg, and each brush contains from 50 to 100 set£e, 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 



