This is another great image of the head of C. elegans, this time showing glutamatergic neurons. Glutamate is a neurotransmitter that is primarily excitatory and is actually the most abundant excitatory neurotransmitter in humans and other vertebrates. Note that I say it is primarily excitatory- like many neurtransmitters, glutamate’s actions are dependent on the receptor it will attach to- and the effects of these receptors can be excitatory or inhibitory. Most receptors for glutamate have an excitatory effect on the post-synaptic neuron, but others have an inhibitory effect (though with glutamate, most of the time, it is excitatory). This idea holds true for most neurotransmitters- they can be either excitatory or inhibitory depending on the receptor subtype they will bind to.
Glutamate is also critical for learning and memory, which I will talk about at another time. For more about C. elegans, go here.