VOLTZOW, J.*; AHMADZADEH, B.; BROWN, R.; Univ. of Scranton, PA: Collagen-wrapped bundles of smooth muscle: A synapomorphy for the Mollusca?

Each phylum of animals can be distinguished by features that are unique to that group. For example, each member of the Chordata has a notochord, a dorsal nerve cord, pharyngeal gill slits, and a post-anal tail. Molluscs, however, are unusual in that there is no similar suite of features that are found in all groups. Although the monophyly of the phylum is generally accepted, each class has only some but not all of the characteristic molluscan features. A bivalve, for example, has a foot and a shell but lacks the characteristic molluscan feeding structure called a radula. A closer examination of the histology of molluscan tissue reveals a distinctive feature of all molluscan groups: bundles or individual smooth muscle fibers are wrapped in sheaths of collagenous connective tissue. This organization gives molluscan tissue its tough texture and permits its tremendous range of movement and flexibility. Comparative histological studies of the musculature and connective tissue of taxa proposed as sisters to Mollusca, including Sipuncula, Annelida, and Arthropoda, indicate that the organization of smooth muscle wrapped in collagenous sheaths is unique to molluscs.