The Ventral and Respiratory structure
Question a
Question b.
Generally, molluscs have a pair of gills and sometimes just one singular gill, but the Polyplacaphora (this type of specimen) they have entire rows of interlocking gills hanging from the roof of the pallial groove along each side of their foot as depicted the diagram above. The respiration in the polyplacahora is facilitated by ctenidia (gill), as indicated in the drawing above. These are the comb-like outgrowths of the mantle and are located within the mantle cavity of the Polyplacaphora, also known as chitons. Each molluscan gill consists of a long flattened axis hangs from the anterior wall of the mantle cavity (Khusbo, 2010)
Further, chitons do have hemoglobin proteins called myoglobin. Though not visible in the drawing above, they are associated with delivering oxygen throughout their body. (“Chiton Glaucus” 2009)
It is important to note that the number of gills varies from species to species. In Polyplacophora, the gills vary 6-80 pairs. A distinct difference is seen when it comes to monoplacophora as the gills vary from 3-6 pairs. For other classes such as the gastropods, there exists either two bipectinate gills or single bipectinate gills. (“Phylum Molusca” 2010)
The location of the gills presents a pertinent difference compared to other classes of molluscus. As presented in the above drawing, the gills are located within the mantle cavity of the polyplacaphora. This type of ctenidia is known as the Holobranchiate type. Found in the polyplacophora, the ctenidia extends all over the body, and like the above specimen, it extends all over the mantle cavity. Differing from this class, the bivalves mollusks have the bipectinate type, which is characterized by flattened gill filaments areranged in two rows. (Nicole and David, 2011). There is a distinct difference observed in the cephalopods whereby they have feathered ctenidia.
References
Nicol, D. (1969). The Number of Living Species of Molluscs. Systematic Zoology, 18(2), 251. https://doi.org/10.2307/2412618
Respiratory Organs of Phylum Mollusca and Its Modification. (2016, May 27). Biology Discussion. http://www.biologydiscussion.com/invertebrate-zoology/phylum-mollusca/respiratory-organs-of-phylum-mollusca-and-its-modification/33024