FURTHER PHYLOGENETIC CONSIDERATIONS
Molluscs are over 500 million years old according to fossil record. There are different views about the evolution of molluscs.
The molluscs have protosotme ancestry. But zoologists do not know the exact relationship of this phylum with other animal phyla. There are following evidences which support protosotme ancestry of molluscs.
1. A mollusc Neopilina (Monoplacophora) was discovered in 1952. It helped greatly to determine the position of molluscs. Neopilina has segmental arrangement of gills, excretory structures, and nervous system. •IIhe annelids and arthropods also have a segmental arrangement of body parts. Theretbre, monoplacophorans are “missing link” between other molluscs and the annelid-arthropod evolutionary line.
2. The molluscs, annelids, and arthropods share certain protosotme characteristics
3. Chitons also show a repetition of some body parts.
Origin form triploblastic stock
Following characteristics does not support the protosotme ancestry of molluscs
1. But the segmentation in some molluscs is very different from annelids and arthropods
2. The serially repeating structure in any other mollusc does not develop like annelid-arthropod.
3. Segmentation is also not an ancestral molluscan characteristic.
Therefore many zoologists now believe that molluscs diverged from ancient triploblastic stocks. Other zoologists still believe that molluscs are tied to the annelid-arthropod line. But it is clear that the relationship of the molluscs to other animal phyla is distant.
History of evolution of molluscs
A diversity of body forms and lifestyles is present in this phylum. Therefore, mollusca are an excellent example of adaptive radiation. Molluscs began in Precambrian times. They were slow-moving, marine bottom dwellers. Then unique molluscan features deli–eloped. It allowed them to diversify quickly. By the end of the Cambrian period, some we .e filter feeders, some were burrowers. and others were swimming predators. Later, some molluscs became terrestrial. They adapted in many habitats like tropical rain forests to arid deserts.
The classes Caudofoveata and Aplacophora lack shell. The zoologists believe that the lack of a shell in a primitive character. All other molluscs have a shell or they are derived from shelled ancestors. The multipart shell distinguishes the Polyplacophora from other classes. They have extensive adaptive radiation. Therefore, it is difficult to develop higher taxonomic relationships of this phylum.