Bilateria Locomotion

Cross references:  Pre-Bilateria Locomotion     Bilateria     
Pre-Chordate Locomotion    Chordates        Early Behavior   
Amphioxus Locomotion
Lamprey Locomotion    Salamander Locomotion    

The hydra is capable of very active locomotion, but I don't seem to have a reference for it at the moment.  

Further study suggests that swimming originally evolved as an escape behavior and was only used later by the amphioxus as a means for making its breeding more efficient. 

Neuromodulation  says: 
An example of one such polymorphic
Central Pattern Generator (CPG) is a multifunctional network of the mollusk Tritonia diomedea. As described by Hooper, weak sensory input to the swimming CPG produces reflexive withdrawal, while strong input produces swimming. The dorsal swim interneurons (DSIs) of the circuit release Serotonin to convert to "swim mode," while application of serotonergic antagonists prevents the swim pattern.[1]
    Since Tritonia is not a chordate, this qualifies as a form of 
Pre-Chordate Locomotion  .  It's also interesting that, in this instance, serotonin stimulates rather than inhibits behavior.  
    My comment
Serotonin as a possible hormone. 

Tuning and playing a motor rhythm: how metabotropic glutamate receptors orchestrate generation of motor patterns in the mammalian central nervous system (Goog) 
Full length HTML available online for free. 
    See also: 
Glutamate Metabotropic Receptor  and  
  Amphioxus Glutamate 
from the Abstract:     
    "Repeated motor activities like locomotion, mastication and respiration need rhythmic discharges of functionally connected neurons termed central pattern generators (CPGs) that cyclically activate motoneurons even in the absence of descending commands from higher centres. For motor pattern generation, CPGs require integration of multiple processes including activation of ion channels and transmitter receptors at strategic locations within motor networks. One emerging mechanism is activation of glutamate metabotropic receptors (mGluRs) belonging to group I, while group II and III mGluRs appear to play an inhibitory function on sensory inputs. Group I mGluRs generate neuronal membrane depolarization with input resistance increase and rapid fluctuations in intracellular Ca2+, leading to enhanced excitability and rhythmicity. While synchronicity is probably due to modulation of inhibitory synaptic transmission, these oscillations occurring in coincidence with strong afferent stimuli or application of excitatory agents can trigger locomotor-like patterns. Hence, mGluR-sensitive spinal oscillators play a role in accessory networks for locomotor CPG activation. In brainstem networks supplying tongue muscle motoneurons, group I receptors facilitate excitatory synaptic inputs and evoke synchronous oscillations which stabilize motoneuron firing at regular, low frequency necessary for rhythmic tongue contractions. In this case, synchronicity depends on the strong electrical coupling amongst motoneurons rather than inhibitory transmission, while cyclic activation of KATP conductances sets its periodicity. Activation of mGluRs is therefore a powerful strategy to trigger and recruit patterned discharges of motoneurons. "  
    My comment:   
Glutamate as a neurotransmitter. 

Neural bases of goal-directed locomotion in vertebrates--an overview
The different neural control systems involved in goal-directed vertebrate locomotion are reviewed. They include not only the central pattern generator networks in the spinal cord that generate the basic locomotor synergy and the brainstem command systems for locomotion but also the control systems for steering and control of body orientation (posture) and finally the neural structures responsible for determining which motor programs should be turned on in a given instant. The role of the basal ganglia is considered in this context. The review summarizes the available information from a general vertebrate perspective, but specific examples are often derived from the lamprey, which provides the most detailed information when considering cellular and network perspectives."  
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