Invertebrate Sensory Receptors

Invertebrate sensory receptors are : (invertebrates sensory structures crossword)

  • (a)baroreceptors
  • (b) chemoreceptors
  • (c) georeceptors
  • (d) thermoreceptors
  • (e) phonoreceptom
  • (f) photoreceptors
  • (g) proprloceptors
  • (h) tactile receptors
  • (i) hygroreceptom

(a) Baroreceptom sense changes in pressure. Zoologists, although have not identified any specific baroreceptors • in invertebrates, yet responses to pressure changes have been identified  in  ocean dwelling copepod crustanceans, ctenophores, jelly fish medusae, and squids.

(b)Chemoreceptors respond to chemicals. This is the oldest and most universal sense in the animal kingdom. Protozoa avoid strong acids, alkali, and salt. Specific chemicals  attract predatory ciliates to their prey. Chemoreceptors of many  aquatic invertebrates are located in pits or depressions. In arthropods, the chemoreceptors called sensilla are usually on the antennae, mouthparts, and legs. Invertebrates with their specific chemoreceptors can detect humidity, access pH, track their prey, recognize food, and locate mate. For example, male silkworm (Bombyx mod) can detect one bombykol molecule. (sex   attractant pheromone) in over a trillion molecules of air from several miles down wind. Fig. 2.16


(c)Georeceptom respond to the force of gravity enabling animal to perceive its orientation relative to “up” and “down”. Most  georeceptors are statocysts. An invertebrate statocyst consists of a fluid filled chamber lined with cilia-bearing sensory epithelium; within the chamber is a solid granule called statolith. Any movement of the animal’s body changes the position of the statolith and moves the fluid which alters the sensory epithelium. Fig. 2.17


Statocysts are found in gastropods, cephalopods, crustaceans, nemertines, polychaetes, and scyphozoans. In addition to statocysts, a number of aquatic insects detect gravity from air bubbles trapped in certain tracheal tubes. These air bubbles, like a carpenter’s level, move according to their orientation to gravity, and stimulate sensory bristles that line the tubes.

(d) Thermoreceptors respond to temperature changes. Although specific receptor structures have not been identified, somehow, a heat sensing mechanism draws leeches and ticks to warm blooded hosts, Paramecium, for example, collects in areas where water temperature is moderate, and avoid temperature extremes. Insects, some crustaceans, and the horseshoe crab (Limulus) can also sense thermal variations.

(e)Phonoreceptors, demonstrated only in insects, arachnids, and centipedes, respond to sound. Crickets, grasshoppers, and cicadas possess phonoreceptors called tympanic or tympanal organs. Most animals possess  phonoreceptors called slit sense organs in their cuticle, centipedes have organs of Tomosvary, both poorly understood. Fig. 2.18




(f)  Photoreceptors   are sensitive  to  light.  All photoreceptors     possess photosensitive    pigments, such as  carotenoids, rhodopsin, which absorb photons of light and produce a generator potential. Among invertebrates, four kinds of photoreceptors are identified: stigma, ocelli, compound eyes and complex camera eyes. Stigma is a mass of bright red photoreceptor granules which are carotenoid pigments which in Euglena perhaps serves as a shield. The actual photoreceptor is the swelling at the base of flagellum. (a in fig. 2.19). °coitus is Si,t7rnulticellular, small cup lined with light sensitive receptors and backed by light absorbing pigment (b in fig.2.19).

The fight sensitive cells are called retinular cells that contain photoreceptor pigment which on receiving stimulus (light) produce generating potential leading to action potential. The animal only gets information about light, but image is not formed. Ocelli commonly occur in phyla Annelida, Mollusca, and Arthropoda. (Earthworm (Lumbricus) has simple unicellular photoreceptor cells scattered over the epidermis). Compound eyes, although occur  in some annelids and bivalve molluscs; they are best developed in arthropods. Each eye consists of a few to many distinct units called ommatidia (ommato, eye + ium, little), each oriented in a slightly different direction from the others due to eye’s convex shape. The visual field of eye is very wide. Compound eyes are very effective in detecting movements and forming image and can detect colour. Colour vision is particularly important in active, day-flying, nectar feeding insects, such as honeybee. Fig. 2.19


Complex camera eyes, such as of squids and octopuses are; best image forming eyes among the invertebrates, largest among animal’s, exceeding 38 cm in diameter in giant squid. (Fig. 2.19 c,d).

(g)    Proprioceptors, commonly called as “stretch receptors” are internal sense organs that respond to mechanically induced changes caused by stretching, compression, bending, or tension. Proprioceptors have been studied thoroughly in arthropods, where they are associated with appendage joints and body extensor muscles.

(h)    Tactile receptors are touch receptors derived generally, from modifications of epithelial cells associated with sensory neurons. Examples include various bristles, spines, setae, and tubercles. For example web-building spiders have tactile receptors that can sense vibrations of the web threads when an entangled prey tries to escape from the web.


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