TISSUES 43 
In some Ccelentera a few of the external cells seem to combine 
contractile and nervous functions. Therefore they are sometimes called 
** neuro-muscular.” 
But in Hydra there are superficial sensory cells, whose basal pro- 
longations are connected either directly with contractile cells, or with 
deeper ganglion-cells, some of which give off motor processes to the 
contractile cells. 
In sea-anemones and some other Ccelentera there is a more sharply 
defined division of labour. Superficial sensory cells are connected 
with subjacent nerve- or ganglion-cells, from which fibres pass to the 
contractile elements. 
In higher animals the sensory cells are mostly integrated into sense 
organs, the ganglionic cells into ganglia, while the delicate fibres. 
which form the connections between sensory cells and ganglionic cells, 
and between the latter and muscles, are represented by well-developed 
nerves. 
So far as we know, nervous tissue always arises from the outer or 
ectodermic layer of the embryo, as we would expect from the fact that 
this is the layer which, in the course of history, has been most directly 
subjected to external stimulus. 
Let-us consider first the ganglionic cells’ which receive stimuli and 
shunt them, which regulate the whole life of the organism, and are the 
physical conditions of ‘‘ spontaneous” activity and intelligence. They 
are of very varied shape, but consist always of a cell-body which gives 
off one or more processes. One of these processes is long, branches 
very sparingly, and is known as the axis-cylinder. There are usually 
present other processes which ramify like the branches of a tree and 
are called dendrites. The cell-body contains a nucleus, distinct 
granules, and a network of fine fibrils. The nervous system is built up 
of such ‘‘neurones.” In the ganglia they are supported and held 
apart by much-branched neuroglia cells. 
In all but a few of the simplest Metazoa, the nerve fibres (axis- 
cylinders) are surrounded by a sheath called the neurilemma, said to be 
formed by adjacent connective tissue. Several nerve fibres may com- 
bine to form a nerve, but each still remains ensheathed in its neuri- 
lemma while fibrous sheaths bind the nerve fibres together. In Verte- 
brate animals each nerve fibre usually has in addition a medullary 
sheath. But even in the higher Vertebrates, ‘‘non-medullated” or 
simply contoured nerve fibres are found in the sympathetic and olfactory 
nerves, and this simpler type alone occurs in hag, lamprey, ‘and 
lancelet, as well as in all the Invertebrates with distinct nerves. 
A nerve fibre contains numerous fibrils like those seen within a 
ganglion cell. These are regarded by some-as the essential elements in 
conducting stimuli, while others maintain that the essential part is the 
less compact, sometimes well-nigh fluid stuff between the fibrils, or that 
the fibrils are but the walls of tubes within which the essentially nervous 
stuff lies. 
The nerve fibres arise as prolongations of the ganglion cells, 
which extend themselves in the embryo like Amoebze sending out 
pseudopodia. 
