ANIMAL — NERVOUS SYSTEM. 
137 
protoplasm, which with age acquire a fusiform, stellate or rounded 
form, but vary in aspect in the different regions of the body, and 
may become firmer, more compact 
and rigid, and constitute skeletal 
tissue, which suiTounds or is dis- 
tributed amongst the muscular and 
epithelial tissues, imparting con- 
sistency and strength, and con- 
stituting the supporting fi’amework 
of the various organs. 
The organs of the body may he 
broadly classified according to their 
function and structure under six chief heads or systems, viz. : — The 
Xervous or Sensitive, the Alimentary or X'utritive, the Circulatory or 
Vascular, the Secretory or Glandular, the IMuscular or ]\lotor, and 
the Sexual or Reproductive systems. 
The Xervous system, upon which all perceptive sensation depends, 
is concentrated in paired ganglionic masses in correlation with the 
bilateral aiTangement of many of the organs of the body, and may be 
considered to be composed typically of four gvoups of more or less 
distinctly paired neiwous masses or ganglia, viz. The Cerebral, the 
Pedal, the Visceral or Parieto-splanchnic, and the Buccal or Stomato- 
gastric. The Cerebral ganglia, which are placed above the oesophagus 
and innervate the head and its organs, are chiefly sensory in function ; 
the Pedal, which are suboesophageal in position and innervate the foot, 
are more especially motor ; while the Parieto-splanchnic and the 
Stomato-gastric centres, which are also situate or connected beneath 
the alimentary canal, and innervate the viscera, are to a certain extent 
analogous in function with the sympathetic system in mammals. 
These various ganglia or nerve centres are constituted by a sui)er- 
hcial or external layer of ganglion cells, which, according to Solbrig, 
have no proper membrane and are chiefly unipolar, with long, branching 
fibrillar extensions or processes, whose aggregation forms the central 
part or nucleus of each ganglion, and also pass into the nervous cords 
by which the various ganglia are connected together. Great varia- 
tion exists in the degvee of approximation of the different ganglionic 
masses ; sometimes by specialization they become fused together 
and form a nerve-ring around the oesophagus, while in other groups 
the constituent ganglia may he variously combined together or widely 
Fig. 292.— Chitinous and 'connective tissue 
framework of the outer ctenidium Anodonta 
anatina, as e.vample of skeletal tissue, w. 
transverse muscular fibres (after Vogt and 
Vung). 
