Mollusks have a dramatic variety of form, ranging from large predatory squids and octopus, some of which show a high degree of intelligence, to grazing forms with elaborately sculpted and colored shells. This phylum can be segregated into seven classes: Aplacophora, Monoplacophora, Polyplacophora, Bivalvia, Gastropoda, Cephalopoda, and Scaphopoda. Figure 2. This chiton from the class Polyplacaphora has the eight-plated shell that is indicative of its class.
These animals lack a calcareous shell but possess aragonite spicules on their epidermis. They have a rudimentary mantle cavity and lack eyes, tentacles, and nephridia excretory organs. The morphology of the shell and the underlying animal can vary from circular to ovate.
A looped digestive system, multiple pairs of excretory organs, many gills, and a pair of gonads are present in these animals. The monoplacophorans were believed extinct and only known via fossil records until the discovery of Neopilina galathaea in Today, scientists have identified nearly two dozen extant species.
These animals have a broad, ventral foot that is adapted for suction to rocks and other substrates, and a mantle that extends beyond the shell in the form of a girdle. Calcareous spines may be present on the girdle to offer protection from predators.
Respiration is facilitated by ctenidia gills that are present ventrally. These animals possess a radula that is modified for scraping. Eyespots are absent in these animals. A single pair of nephridia for excretion is present.
Figure 3. These mussels, found in the intertidal zone in Cornwall, England, are bivalves. Members of this class are found in marine as well as freshwater habitats. The overall morphology is laterally flattened, and the head region is poorly developed. Eyespots and statocysts may be absent in some species. These animals are suspension feeders—they eat material, such as plankton, that is suspended in the water around them.
Due to their diet, this class of mollusks lacks a radula. Respiration is facilitated by a pair of ctenidia, whereas excretion and osmoregulation are brought about by a pair of nephridia. Bivalves often possess a large mantle cavity. In some species, the posterior edges of the mantle may fuse to form two siphons that serve to take in and exude water. One of the functions of the mantle is to secrete the shell. This property has been commercially exploited to produce pearls.
Gastropoda includes shell-bearing species as well as species with a reduced shell. These animals are asymmetrical and usually present a coiled shell Figure 4. Shells may be planospiral like a garden hose wound up , commonly seen in garden snails, or conispiral , like a spiral staircase , commonly seen in marine conches. Figure 4. The visceral mass in the shelled species displays torsion around the perpendicular axis on the center of the foot, which is the key characteristic of this group, along with a foot that is modified for crawling Figure 5.
Most gastropods bear a head with tentacles, eyes, and a style. A complex radula is used by the digestive system and aids in the ingestion of food. Eyes may be absent in some gastropods species. The mantle cavity encloses the ctenidia as well as a pair of nephridia.
Figure 5. During embryonic development of gastropods, the visceral mass undergoes torsion, or counterclockwise rotation of anatomical features. As a result, the anus of the adult animal is located over the head. Torsion is an independent process from coiling of the shell. Figure 6. Members of the genus Conus produce neurotoxins that may one day have medical uses.
Nearly all snails grow a spiral shell that is all one piece. A few snail groups have stopped growing shells; they're called slugs. The next largest group is Bivalvia , the clams, oysters, and mussels. These are mollusks with two shells that they can close up tight for protection. Finally there is Cephalopoda , the squids and octopuses. They only live in salt water, so we won't say much about them here. They have no shells, but are larger, smarter, and faster than their relatives in the other groups.
Squids and octopuses are all predators; they eat fish, crustaceans, and other mollusks. Annelids possess a well-developed complete digestive system with specialized organs: mouth, muscular pharynx, esophagus, and crop.
A cross-sectional view of a body segment of an earthworm is shown in [Figure 6] ; each segment is limited by a membrane that divides the body cavity into compartments. Gas exchange occurs across the moist body surface. Annelids have a well-developed nervous system with two ventral nerve cords and a nerve ring of fused ganglia present around the pharynx.
Annelids may be either monoecious with permanent gonads as in earthworms and leeches or dioecious with temporary or seasonal gonads as in polychaetes. This video and animation provides a close-up look at annelid anatomy.
Phylum Annelida includes the classes Polychaeta and Clitellata [Figure 7] ; the latter contains subclasses Oligochaeta, Hirudinoidea, and Branchiobdellida. Earthworms are the most abundant members of the subclass Oligochaeta, distinguished by the presence of the clitellum , a ring structure in the skin that secretes mucus to bind mating individuals and forms a protective cocoon for the eggs. The chaetae of polychaetes are also arranged within fleshy, flat, paired appendages on each segment called parapodia.
The subclass Hirudinoidea includes leeches. Significant differences between leeches and other annelids include the development of suckers at the anterior and posterior ends, and the absence of chaetae. Additionally, the segmentation of the body wall may not correspond to internal segmentation of the coelomic cavity. This adaptation may allow leeches to swell when ingesting blood from host vertebrates.
The subclass Branchiobdellida includes about species that show similarity to leeches as well as oligochaetes. All species are obligate symbionts, meaning that they can only survive associated with their host, mainly with freshwater crayfish. They feed on the algae that grows on the carapace of the crayfish.
The phylum Mollusca is a large, mainly marine group of invertebrates. Mollusks show a variety of morphologies. Many mollusks secrete a calcareous shell for protection, but in other species, the shell is reduced or absent.
Mollusks are protostomes. The dorsal epidermis in mollusks is modified to form the mantle, which encloses the mantle cavity and visceral organs. This cavity is distinct from the coelomic cavity, which the adult animal retains, surrounding the heart. Respiration is facilitated by gills known as ctenidia.
A chitinous scraper called the radula is present in most mollusks. Mollusks are mostly dioecious and are divided into seven classes. The phylum Annelida includes worm-like, segmented animals.
Segmentation is both external and internal, which is called metamerism. Annelids are protostomes. The presence of chitinous hairs called chaetae is characteristic of most members.
These animals have well-developed nervous and digestive systems. Polychaete annelids have parapodia that participate in locomotion and respiration. Suckers are seen in the order Hirudinea. Breeding systems include separate sexes and hermaphroditism. Mollusks have a large muscular foot that may be modified in various ways, such as into tentacles, but it functions in locomotion. They have a mantle, a structure of tissue that covers and encloses the dorsal portion of the animal and secretes the shell when it is present.
The mantle encloses the mantle cavity, which houses the gills when present , excretory pores, anus, and gonadopores. The coelom of mollusks is restricted to the region around the systemic heart. The main body cavity is a hemocoel.
Many mollusks have a radula near the mouth that is used for scraping food.
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