258 Elementary Biology. 



Locomotory, protective, and supporting systems. 



Protection is afforded rather by the habits of the animal 

 than by any hardened integument such as we find among 

 fish or Crustacea. Amphioxus live, as a rule, buried in 

 sand below water ; a skin specially suited to perform a 

 protective function is therefore unnecessary. The body is 

 well provided wich muscles. They are in the form of 

 myotomes, or long thick plates, separated by fibrous septa. 

 The muscle fibres themselves are arranged horizontally in 

 the long axis of the body, so that the characteristic fish like 

 motion is brought about by alternate contraction of the 

 muscle bands, first on one side, then on the other. A sheet 

 of transversely placed muscle fibres also stretches along the 

 floor of the atrial cavity, the duty of which is to expel the 

 water from the atrial cavity. 



It is in Amphioxus that we first meet with rudiments of 

 the skeleton in the interior of the body,- which becomes so 

 fully developed in the higher forms of animal life. It 

 could not well be simpler than it is in Amphioxus, for it 

 consists only of a delicate rod of clear thin-walled cells lying 

 immediately above the pharynx and alimentary canal. It 

 has been called the notochord. The notochord is enclosed 

 by a sheath of fibrous tissue, and from its nature is highly 

 elastic. The notochord, it is important to note, extends 

 throughout the entire length of the body. In addition to 

 this axial rod the lips of the buccal cavity are strengthened 

 by delicate bars, from which arise yet finer bars to support 

 the tentacular processes. The branchial bars may also be 

 looked upon as part of the internal skeleton ; whilst the 

 entire body is strengthened by a considerable development 

 of dense fibrous tissue which surrounds the notochord, the 

 nervous system (immediately dorsal to the notochord), 

 separates the muscle plates from each other, and strengthens 

 the bases of the fins. 



The nervous system. The nervous system is dorsally 

 placed, and supported beneath by the notochord. It con- 



