Baroreceptors and chemoreceptors:
baros = weight + receptor. Baroreceptors sense changes in pressure. Responses to pressure changes have been identified in ocean dwelling copepod crustaceans, ctenophores, jelly-fish medusa and squids. Some intertidal crustaceans coordinate migratory activity with daily tidal movement possibly in response to pressure changes accompanying water depth changes.
Chemoreceptors: They pertain to chemistry and respond to chemicals. Protozoa have chemical sense. They respond with avoidance behaviour to acid, alkali and salt stimuli. Chemoreceptors of may aquatic invertebrates are located in pits or depression through which water carrying the specific chemicals may be circulated. In arthropods chemoreceptors are on antennae, mouthparts and legs in the form of hollow hairs containing chemo sensory neusons. The types of chemicals which invertebrates respond are closely associated with their life styles as humidity detection, pH assessment, prey tracking, food recognition and mate location.
Statocyst:
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.
Statocysts consist of a fluid filled chamber lined with cilia bearing sensory epithelium, within the chamber is a solid granule called a statolith (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. 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.
Hygroreceptors and Phonoreceptors:
Hygroreceptors: Hygro = moist. They detect the water content of air e.g. some insects have hygrorecepros that can detect small changes in the ambient relative humidity. This sense enables them to seek environment with specific humidity or to modify their physiology or behaviour with respect to ambient humidity (to control the opening or closing of spiracles). These structures are found on antennae, palps, under side of body and near spiracles of insects.
Phonoreceptors: Phono = voice. They respond to sound and are demonstrated only in insects, arachnids and centipedes, although other invertebrates seem to respond to sound induced vibrations of the substratum e.g. crickets, grasshoppers and cicadas possess phonoreceptors called tympanic or tympanal organs. This organ consists of a flexible tympanum that covers internal sac that allows the tympanum to vibrate when sound waves strike it. Sensory neurons attached to tympanum are stimulated and produce generator potential.
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