122 GENERAL PRINCIPLES OF ZOOLOGY. 



with the skin. If a skeleton be formed by the skin, as in the 

 arthropods, then the sac breaks up into groups of muscles, which 

 find points of attachment upon the dermal skeleton; if, on the 

 other hand, as in the vertebrates, an axial skeleton be formed, a 

 fixed point is furnished for muscular action, so that the muscula- 

 ture obtains a quite new character, in particular a deeper position. 

 A locomotor apparatus quite unique is the ambulacra! system of 

 the echinoderms, a system of delicate little tubes with protrusible 

 portions which function as feet, described in connexion with that 

 group. 



II. Nervous System. 



Scarcely a system of organs in the animal series shows such a 

 regular development as the nervous system. The different stages 

 which can be grouped may be termed the diffuse, the linear, the 

 ganglionic, and the tubular types. 



Diffuse Nervous System. The diffuse type is certainly the 

 most ancestral; it shows the two elements, nerve fibres and 

 ganglion cells, regularly distributed through the whole body, or, 

 at least, through certain layers of the body. The skin of the 

 body, the ectoderm, is to be looked upon as one of the fundamental 

 elements in the nervous system, since it is related to the external 

 world, and hence receives the sensory impressions, so important 

 for the development of nervous tissue. The corals and hydroid 

 polyps are examples, since in them the ectoderm is permeated in 

 all directions by a delicate, subepithelial spider-weblike network 

 of nerve fibres and ganglion cells, which encroach even upon the 

 entoderm. 



Linear Nervous System. From the diffuse type the other 

 chief types can be derived through concentration, which is chiefly 

 conditioned by the fact that there are a few points which are most 

 advantageously located for the reception of sensory stimuli, and 

 hence for the development of nervous elements. In the medusae 

 such a place is the rim of the bell; consequently a stronger nerve- 

 cord remarkably rich in ganglion cells is found here. This, as 

 well as the nerve-ring and the five ambulacral nerves of echino- 

 derms, may be called a central system, thereby distinguishing the 

 rest of the nervous network as the peripheral nervous system. 



Ganglionic Central Nervous System. Numerous transitional 

 forms lead to the ganglionic central nervous system of the worms, 

 molluscs, and arthropods (fig. 74). The central nervous system 

 here consists of two or more ganglia; each ganglion being a 



