Chpt 16 Mollusca

Chpt 16: Mollusca 

Characteristic of phylum Mollusca (snails, clam, octopuses, and  

  • Mantal: encloses mantle cavity and is modified into gills or lungs and secretes the shell (in species that have one)  

  • Visceral mass: contains internal organs  

  • Ventral body wall specialized as a muscular foot 

  • Radula: specialized feeding organ used to scrape food particles from surfaces. Liked a toothed tongue  

  • Anus usually empties into mantle cavity 

  • Live in marine, freshwater, and terrestrial habitat  

  • Mostly free living  

  • Coelom limited to mainly to area around heart, lumen of gonads, part of kidneys 

  • No asexual reproduction. Both monoecious and dioecious. All have trochophore larva some have veliger larva.  

  • One or 2 kidneys that open into the pericardial cavity and empty into the mantle cavity  

  • Gaseous exchange by gills, lungs, mantle, or body surface 

  • Open circulatory system: blood is not contained in blood vessels but bathes the organs directly  

  • Cephalopods (squids and octopuses) have closed circulatory system where blood is confined to vessels  

Torsion: relative position of body parts change in Gastropods 

  • Moves move mantle cavity from the posterior to the front of the body. Twists the visceral organs through a 90- or 180-degree rotation while in the veliger state  

  • The anus moves from posterior to anterior.  

  • End goal is to have both the anus and mantle cavity. Results in fouling, the release of digestive waste over the head and in front of the gills  

Coiling: elongation and spiral of the visceral mass  

  • It occurs at the same larval stage as torsion but has a separate earlier evolutionary origin.  

  • Planospiral shell: all whorls in a single plane 

  • Conispiral: more compactness, each whorl is to the side of the previous one  

  • Clearly unbalanced which led to loss of organs and the shifting of the shell upward and back.  

  • All living gastropods originated from coiled, torte ancestors although some have lost that characteristic.  

Taxonomy of Phylum Mollusca 

 

Class Caudofoveata  

  • Wormlike. No shell, head, or excretory organs. No shell but body covered wit calcareous scales . Radula present but reduced  

  • Chaetoderma. Limifossor  

Class Solenogastres  

  • No radula or gill, Hermaphroditic (both female and male parts), bottom dwellers, live and feed on cnidarians  

  • Neomenia  

Class Polyplacophora (Chiton) 

  • Name means many plates  

  • Flattened bodies, radula present, shell of 7 or 8 dorsal plates, foot is broad and flat, multiple gills along sides of the body between foot and mantle. Separate sexes. Trochophore but no veliger larva 

  • Mopaliid, Tonicella  

Class Monoplaceophora  

  • Name means one plate bearing  

  • Bilateral symmetry, broad flat foot, single limpetlike shell, mantle cavity with 3 to 6 pairs of gills. Large coelomic cavities, radula present, 3 to 7 pairs of nephridia. Separate sexes  

  • Neophilia  

Class Gastropods: Snails and Slugs 

  • Very diverse, heavy shell, slow locomotion. Shell is univalve (one piece).  

  • Apex is the oldest and smallest whorl. Whorls go around the central axis known as a columella.  

  • Trochophore and veliger lobes.  

  • Large flat foot, 1 or 2 gills, nervous system with cerebral, pleural, pedal, and visceral ganglia. Diocious and monoecious, some with trochophores most with veliger.  

  • Busycom, Polimices, Physa, Helix, Aplysia  

Class Bivalvia  

  • Name means 2 valves  

  • Body enclosed in 2 lobed mantles, head reduced, no radula, no cephalic eyes, wedge shaped foot, separate sexes, typically with trochophore and veliger larvae.  

  • Anodonta, Venus, Tagelus  

Class Scaphopoda 

  • Name means: trough foot  

  • Tusk shell, body enclosed in one pieace tubular shell open at both ends. Concical foot, mouth with radula and contactile tentacles. Head absent, mantle for respiration, sexes separated, trochophore larvae.  

  • Dentalium  

Class Cephalopoda (squid, cuttlefish, octopuses)  

  • Reduced shell or absent shell. Well developed head with eye and radula, foot modified into siphon, nervous system of well-developed ganglia, centralized to form a brain, sexes separate  

  • Sepioteuthis, octopus, sepia  

 

Other things to Know:  

  • Ocean acidification is a serious threat to Molluscs because increased acidity makes it harder to secrete calcium which is the key component in a healthy shell. Bivalves are particularly affected y this issue  

  • Basic body division of molluscs is head-feet and visceral mass.  

  • Foot is usually ventral, sole-like, locomotory organ. It can be modified to become arms and a funnel  

  • Mantle secretes the shell and houses the gills. The mantle cavity can be modified to into lungs.  

  • Radula is tongue-like organ with teeth that is used for feeding.  

  • Primary larva of molluscs is the trochophore which develops into the second larval state, the veliger 

  • Most molluscs have a complex nervous system with a variety of organs.