Analogy also produces equally remarkable internal or skeletal transformations.
A prominent and practically important illustration of neural tonus is given by the skeletal muscles.
Rhythmic contraction is also characteristic of certain groups of skeletal muscles, e.g.
The dorsal skeletal elements of the thorax and of the anterior six abdominal segments unite with the wing-cases to form a large respiratory chamber, containing five pairs of tracheal gills, with lateral slits for the inflow and a posterior orifice for the outflow of water.
With certain of these it stands associated most closely, namely, with the vestibular, representing the sense organs which furnish data for appreciation of positions and movements of the head, and with the channels, conveying centripetal impressions from the apparatus of skeletal movement.