ANIMAL — ALIMENTARY SYSTEM. 
las 
separated and distant. Tlie ner\'es arising from these various centres 
innervate all parts of the body and take on detinite duties or functions ; 
those proceeding to the muscles 
are chietly efferent or motor, while 
those terminating in sensory organs, 
whether merely tactile or of a more 
s])ecialized character, are termed 
afferent or sensory. 
The various organs of special 
sense are nervous differentiations 
ada})ted to the various forms of 
percei)tion suitable to the environ- 
ment and habits of the organism, 
hut the whole surface of the body is more or less acutely sensible to 
tactile and other imi)ressions, although perception is more especially 
concentrated in the exposed and prominent i)ortions of the body. 
The Auditory or Equilibrating organs are present in the active forms 
and are alwaj's buried in the tissues of the foot, though innervated 
by the cephalic ganglia, and consist of a pair of closed sacs, termed 
otocysts, lined with ciliated sensorial ei)ithelium and enclo.sing cal- 
careous concretions. The Cephalic E3'es are jiigmented invaginations 
of the integument, usually closed by a layer of epithelium and con- 
taining a ciystalline lens, and receiving their innervation from the 
cephalic ganglia. The skin generally is howevei' dermatoptic and in 
a measure sensible to the iiiHuence of light and shade. The Olfactory 
organs are api)arently intlueneed in their i)Osition by the character of 
the respiratory organs, as they may be located in the cephalic or in 
the pallial region, or he recognizably present in both areas ; they are 
formed by the development and local concentration of neuro-epithelial 
cells, and according to their i»allial or cei)halic i)Osition may he in- 
nervated by the visceral or h}" the sui)ra-oesophageal ganglia. 
The Alimentary system by which nutrition is effected is a more 
or less complicated and diversitied tube, composed chiefly of epithelial 
tissue, and embraces an anterior apertiire or month, guarded by 
external lobes or palps, which leads by the a’SOi)hagus into a stomach 
or crop, into or near to which the usually voluminous liver or digestive 
gland discharges its secretion Ijy suitalde ducts. Occasionally the 
stomach has a c;ecal diverticulum, within which, or in the intestine 
itself, there is a rod of gelatinous consistency, the Crystalline Style, 
Fig. 293. Fig. 291. Fig. 295. 
Ganglion cells of I'nio pktoruw^ highly 
magnified (after Rawitz). Fig. 203 from 
cerebral ganglion ; Fig. 204 from visceral 
ganglion ; Fig. 205 from pedal ganglion. 
