Editing Chaetognatha (section)

===Nervous and sensory systems===
The nervous system is reasonably simple and shows a typical [[protostome]] anatomy,<ref name="Perez2021"/> consisting of a [[ganglion]]ated nerve ring surrounding the pharynx. The brain is composed of two distinct functional domains: the anterior neuropil domain and the posterior neuropil domain. The former probably controls head muscles moving the spines and the digestive system. The latter is linked to eyes and the corona ciliata. A putative sensory structure of unknown function, the retrocerebral organ, is also hosted by the posterior neuropil domain.<ref name="Perez2021"/> The ventral ganglion is the largest, but nerves extend from all the ganglia along the length of the body. 

Chaetognaths have two compound eyes, each consisting of a number of pigment-cup [[ocelli]] fused together; some deep-sea and [[troglobitic]] species have unpigmented or absent eyes.<ref name="Perez2021"/> In addition, there are a number of sensory bristles arranged in rows along the side of the body, where they probably perform a function similar to that of the [[lateral line]] in fish. An additional, curved, band of sensory bristles lies over the head and neck.<ref name=IZ/> Almost all chaetognaths have "indirect" or "inverted" eyes, according to the orientation of [[Photoreceptor cell|photoreceptor]] cells; only some [[Eukhroniidae]] species have "direct" or "everted" eyes.<ref name="Perez2021"/> A unique feature of the chaetognath eye is the lamellar structure of photoreceptor membranes, containing a grid of 35–55 nm wide circular pores.<ref name="Perez2021"/>

A significant mechanosensory system, composed of ciliary receptor organs, detects vibrations, allowing chaetognaths to detect the swimming motion of potential prey. Another organ on the dorsal part of the neck, the corona ciliata, is probably involved in chemoreception.<ref name="Perez2021"/>