8.5 Superphylum Lophotrochozoa: Mollusks and Annelids
8.5 Superphylum Lophotrochozoa: Mollusks and Annelids
Mary Ann Clark; Jung Choi; and Matthew Douglas
Learning Objectives
By the end of this section, you will be able to do the following:
- Describe the unique anatomical and morphological features of mollusks and annelids
- Describe the formation of the coelom
- Identify an important extracoelomic cavity in mollusks
- Describe the major body regions of Mollusca and how they vary in different molluscan classes
- Discuss the advantages of true body segmentation
- Describe the features of animals classified in phylum Annelida
The annelids and the mollusks are the most familiar of the lophotrochozoan protostomes. They are also more “typical” lophotrochozoans, since both groups include aquatic species with trochophore larvae, which unite both taxa in common ancestry. These phyla show how a flexible body plan can lead to biological success in terms of abundance and species diversity. The phylum Mollusca has the second greatest number of species of all animal phyla with nearly 100,000 described extant species, and about 80,000 described extinct species. In fact, it is estimated that about 25 percent of all known marine species are mollusks! The annelids and mollusca are both bilaterally symmetrical, cephalized, triploblastic, schizocoelous eucoeolomates They include animals you are likely to see in your backyard or on your dinner plate!
Phylum Mollusca
The name “Mollusca” means “soft” body, since the earliest descriptions of mollusks came from observations of “squishy,” unshelled cuttlefish. Mollusks are predominantly a marine group of animals; however, they are also known to inhabit freshwater as well as terrestrial habitats. This enormous phylum includes chitons, tusk shells, snails, slugs, nudibranchs, sea butterflies, clams, mussels, oysters, squids, octopuses, and nautiluses. Mollusks display a wide range of morphologies in each class and subclass, but share a few key characteristics (see figure below). The chief locomotor structure is usually a muscular foot. Most internal organs are contained in a region called the visceral mass. Overlying the visceral mass is a fold of tissue called the mantle; within the cavity formed by the mantle are respiratory structures called gills, that typically fold over the visceral mass. The mouths of most mollusks, except bivalves (e.g., clams) contain a specialized feeding organ called a radula, an abrasive tonguelike structure. Finally, the mantle secretes a calcium-carbonate-hardened shell in most mollusks, although this is greatly reduced in the class Cephalopoda, which contains the octopuses and squids.
The muscular foot is the ventral-most organ, whereas the mantle is the limiting dorsal organ that folds over the visceral mass. The foot, which is used for locomotion and anchorage, varies in shape and function, depending on the type of mollusk under study. In shelled mollusks, the foot is usually the same size as the opening of the shell. The foot is both retrac