Phylum Mollusca

Phylum Mollusca is defined by several special characteristics.  Their bodies are divided into three regions, head, visceral mass and ventral foot.  Often observed in Mollusca organisms, the body is encased by a mantle and shell.  There are variations of this aspect; the Flamingo Tongue is a fascinating organism, in that their visceral mass, when they are at rest, encompasses the mantle and shell exteriorly.  When threatened the Flamingo Tongue will retract all of the exposed tissue back into the mantle in a very peculiar manner.  Mollusca are adaptable in that may be found in both aquatic and terrestrial habitats.  More features of Mollusca include but are not limited to: bilateral symmetry, soft/ unsegmented bodies, respiration via ctenidia, ganglia/nerve comprised nervous system, haemocoel body cavity, etc. [1]. One human valued aspect of Mollusca can be found in the species that produce pearls!

            The Chiton seen below is the common term for polyplacophorans which is an overarching class among the phylum Mollusca. Chitons are considered more primitive in relation to other diverse groups within the phylum of Mollusca such as bivalves or cephalopods. There are 8 dorsal plate lining the ‘backs’ of these organisms.  Interestingly enough, there are even more primitive species of these organisms still alive today.  Few species of monoplacophorans still exist in the world today.  See the anatomical illustration in the second figure below for more detail on the phenotypic traits often exhibited in these species.  Notice the relatively immense stature of the foot in relation to the rest of the organism’s body.  This is indicative of this appendage’s importance to these organisms function and survival.

Credit: [1]
Picture taken under discection-scope of dorsal side of chiton specimen

           Gastropods such as the one depicted below is a Prosobranchia, which are often identified by their coiled, cone shaped or tubular shells [1].  The mantel cavity is typically located to the anterior of the organism.  Important distinctions of these organisms are variations or the absence of a radula.  Radula being a saw-mill like structure on the interior of the mouth, superior to Odontophore.  The function of this structure is to scrape the substrate the organism is attached to, thereby collecting nutrients on the substrate.  Opposite from the mouth these organism have Nephridium, analogous to the function of an anus.  The Nephridium is the orifice responsible for the excretion of waste [4].  One of the most defining characteristics of these gastropods is the presence of a calcareous operculum.  This structure, when the organism is exposed and mobile, seems as a shielded plate, at the posterior of the organism.  When the organism retracts into its mantel for protection when threatened, the operculum serves as a protective cap to the opening of the shell.

Notice the dark, oval shaped structure in the middle, this is the operculum! There are calcernious and rough to the touch! Protecting that vital opening to the shell like a man-hole cover.
Credit: [1]

            Nudibranchs are generally categorized as sea snails that are absent shells.  Often they are richly colored and captivating to the eye.  In addition to their magnificent beauty, Nudibranchs are regarded for their interesting behavior.  Much of this lies within their aspects of aposematism, the bright and variant colorations exhibited by these organisms are in fact warning signs with a punch to back it up.  The toxicity of these organisms differentiates from species to species, with some being exponentially more dangerous than others, usually contingent upon their evolutionary specialization and the niche that the specific genus fits.  A little-known fact of Neogastropods is that they have a heart, exclusively with a left atrium! 

Credit: National Geographic

            Class Bivalvia consists of Oyster, Clams, Mussels, Scallops and more!  Many are enjoyed as delicacies, despite the irony in that they are benthic level filter-feeders.  Despite a seemingly simplistic anatomical makeup, these organisms are intricate and unique in their makeup.  Along with all Mollusca, bivalves have a shell that is made up of deposits of Calcium Carbonate.  These deposits are derived from substances in the water and harden over-time.

Picture of specimen shells taken during lab

            The diagram below of clam anatomy is a perfect example of the statement above concerning these organisms complexity.  Visible among the many dozen aspects of the diagram are the posterior and anterior adductor muscles.  The function of these muscles is to hold closed the mantel shell, as a defense from potential predators.  The strength of these muscles given their relative size is immense; anyone who has ever tried to shuck a clam or oyster can attest to this!  The mantel depiction, is a layer of tissue that overlays the visceral mass of these organisms and is directly connected to the shell.  The large appendage that is depicted in the diagram is the foot of the organism, which is directly responsible for movement.  This appendage will emerge when the shell is opened and pushes the organism along the benthic level of the ocean. 

            Cephalopods are variant among Mollusca, exhibiting several similarities but also distinguishable differences.  They may and or may not exhibit a Calcium Carbonate shell.  Squids and Octopi are lacking in this feature, but Nautili do show this characteristic.  Cephalopods contain a largely closed circulatory system, prehensile arms/ tentacles that encompass a mouth, complete with beak and radula.  One of the most important evolutionary aspects of these organisms are large complex eyes, specialized for improved sight at depths where little ambient light reaches.  The mantel of these organisms forms a sizable ventral pallial cavity with ctenidia, complete with a muscular funnel.  Water is up-taken and forced through these chambers under pressure, compiling a unique form of jet propulsion.  See diagram below for more.  There are over 900 living species of cephalopods that inhabit the world today.

Credit: [1]
Shell specimen of what was likely once home to a Nautilus; picture taken during lab
Credit: [1]

            More information may be found at your local library, inside of an invertebrate biology textbook!  Also an excellent investment or gift for any scholar, highly recommended!

References

  1. Brusca, Gary J., Brusca, Richard C. 2003, Invertebrates 2nd ed.  ISBN 0-87893-097-3
  2. The Editors of Encyclopaedia Britannica 2019, via Enclyclopaedia Briannica
  3. Horne, F. (2019).  Texas State Univertisy. How are seashells created? Or any other shell, such as a snail’s or a turtle’s? Retrieved from https://www.scientificamerican.com/article/how-are-seashells-created/
  4. Phylum Mollusca: Characteristics, Classification, Examples with Videos. (2018, September 19). Retrieved from https://www.toppr.com/guides/biology/animal-kingdom/phylum-mollusca/

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