Baroreceptors:
Baroreceptors (Baro=weight + receptor) sense changes in pressure. Responses to pressure changes have been identified in ocean dwelling copepod crustaceans, ctenophore, jelly fish medusae and squids. Some intertidal crustaceans coordinate migratory activity with daily tidal movement possibly in response to pressure changes accompanying water depth changes.
Chemoreceptors:
Chemo=pertaining to chemistry. They respond to chemicals. Protozoa have a chemical sense. They respond with avoidance behaviour to acid, alkali and salt stimuli. Specific chemical attract predatory ciliates to their prey. Chemoreceptors of many aquatic invertebrates are located in pits or depressions through which water carrying the specific chemicals may be circulated. In Arthropods chemoreceptors are usually on the antennae, mouth parts and legs in the form of hollow hairs containing chemo sensory neurons. The types of chemicals which invertebrates respond are closely associated with their life styles e.g. humidity detection, pH assessment, prey tracking, food recognition and mate location. The antennae of male silk worm (Bombyx mori) can detect one mbobykol molecule in over trillion molecules of air. Female silk moths secrete bombykol as a sex attractant which enables a male to find female at might from several miles down wind, an ability that confers obvious reproductive advantage in a widely disappeared species.
Statocysts:
Statocysts are found in various gastropods, cephalopds, crustaceans, nemertines, polychaetes and sayphozoans. These animals use information from statocysts in different ways. For example burrowing invertebrates cannot rely on photoreceptors for orientation instead, they rely on georeceptors for orientation within the substation. Most georeceptors are statocysts (statos=standing + kystis=Bladder). Statocysts consist of a fluid filled chamber lined with cilia bearing sensory epithelium, within the chamber is a solid granule called a statolith (lithos=stone). Any movement of the animal changes the position of the statolith and moves the fluid, thus altering the intensity and pattern of information arising from the sensory epithelium.
For example when an animal moves both the movement of the statolith and the flow of fluid over the sensory epithelium provide information about animals linear and rotational acceleration relative to the environment.
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