A. Phylum Mollusca Is the Second Largest Animal Phyla After Arthropoda
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a. Phylum Mollusca is the second largest animal phyla after Arthropoda. There are over 90,000 described living species and some 70,000 fossil species. b. Their name comes from the Latin word ‘molluscus’ meaning soft, after their soft bodies. Although in many species, their soft body is protected by a shell. c. Mollusks are a diverse group that range from fairly simple organisms to some of the most complex and specialized of the invertebrates. They include chitons, snails, slugs, nudibranchs, sea butterflies, clams, mussels, oysters, scallops, cuttlefish, squid, octopuses, and nautiluses. d. Mollusks are found in a great range of habitats, although most require water. Only snails and slugs are terrestrial, yet they too are limited in their range by their need for humidity, shelter, and presence of calcium. e. Mollusks are triploblastic coelomate protostome invertebrates. i. While the molluscan coelom is relatively small, the development of the coelom was a major step in the evolution of larger and more complex forms of animals. a. There are seven living classes of Mollusks: i. Polyplacophora 1. Contains 600 species of chitons found on rocky marine shorelines. ii. Gastropoda 1. Contains an estimated 40,000 - 150,000 species of nudibranchs, snails and slugs. Also includes unique species such as limpets, sea hares, sea angel, sea butterfly, and Sea Lemon (pictured above). iii.Bivalvia 1. Also called Pelecypoda; contains 8,000 species of clams, oysters, scallops, and mussels. iv.Cephalopoda 1. Contains 786 species of squid, octopus, nautilus, and cuttlefish. v. Monoplacophora, Aplacophora and Scaphapoda are rare and/or extremely deep-water creatures, and will not be discussed. a. Mollusks vary in appearance (think of snails, clams, oysters, octopuses, squid, etc.) but all share the following general characteristics: i. Most mollusks have bilateral symmetry, and many have one or more shells called valves. ii. Three-part body plan 1. Most mollusk bodies are composed of three main parts, the head, foot, and visceral mass. a. The head-contains the mouth and a variety of sensory structures i. Most mollusks have well-developed heads, which contain their mouth, photosensory receptors, and a radula. ii. Photosensory receptors range from fairly simple ones to the complex eyes of cephalopods. 2. The radula is a rasping, tongue-like organ that is found in all mollusks except bivalves. a. It is used for feeding and consists of a ribbon-like membrane that has rows of teeth that point backward. b. This radula scrapes fine particles of food off of hard surfaces. Then it acts as a conveyer belt and takes the food to the mouth of the mollusk. It can also be used as a weapon to puncture prey. c. As the radula wears away anteriorly, new rows of teeth are continuously replaced by secretion at its posterior end. b. The foot is the large, muscular organ used for locomotion and prey capture. i. It varies in shape and function, depending on the type of mollusk. ii. Tentacles are often present as part of the foot. c. The visceral mass contains the body’s organs. i. Surrounding the visceral mass is a membrane called the mantle. 1. The mantle is a sheath of skin extending from the visceral mass that hangs down on each side of the body, protecting the soft parts 2. The outer surface of the mantle secretes the shell. 1. The space between the mantle and the visceral mass is called the mantle cavity. 2. The mantle cavity plays an important role in the life of a mollusk. It usually houses respiratory organs (gills or lung). 3. Products from the digestive, excretory, and reproductive systems are emptied into the mantle cavity to be flushed out of the body. 4. In cephalopods (squids and octopuses) the muscular mantle and its cavity create jet propulsion used in locomotion. Many mollusks can withdraw their head or foot into the mantle cavity, which is surrounded by the shell, for protection. 1.The protective shell of a mollusk is secreted by the mantle. 2.The shell first begins to form during the embryo stage and grows throughout the life of the mollusk, as shown by growth lines or rings on the shell. 3.The shell is composed of 3 layers. a.The outer layer is called the periostracum. i. This layer is especially thick in freshwater species to protect from acidity. ii.This layer is thin or absent in many marine forms. b.The middle layer is called the prismatic layer. i. This is made up of calcium carbonate in a protein matrix. ii.Calcium to make the shell is extracted from water, soil, or food. c. The inner layer is called the nacreous layer, made of nacre. i. This layer is secreted continuously by the mantle surface. This allows for this layer to increase in thickness during the life of the animal. ii.The calcareous nacre is laid down in thin layers. These thin layers produce the iridescent, “mother-of-pearl” rainbow shine found in nautiluses and bivalves. 4. In some such as squid and cuttlefish the shell is internal and greatly reduced; in this case the mantle serves as the animals’ outer covering. a. Most of the internal organs are embedded the in solid tissue of the visceral mass, instead of being free inside a body cavity. i. The digestive tract is complex and highly specialized, according to feeding habits of the various mollusks. ii. The foregut or buccal cavity includes the mouth, radula, salivary glands, and esophagus. It receives and prepares food. iii. The midgut contains the stomach and associated digestive glands used for sorting and crushing food. iv. The hindgut, or intestine, is usually long and coiled for the absorption of nutrients, followed by the formation of feces (undigested residue). i. The nervous system consists of several pairs of ganglia with connecting nerve cords, and it is generally simpler than that of annelids and arthropods. ii.There are various types of highly specialized sense organs. i. Mollusks are the oldest known animals to have evolved excretory structures called nephridia. Nephridia are organs that remove metabolic wastes from the animal’s body. Most mollusks have a pair of nephridia called metanephridia. i. Most mollusks have an open circulatory system with a pumping heart, blood vessels, and blood sinuses. 1.In an open circulatory system blood is not entirely contained within blood vessels; rather it flows through vessels in some parts of the body and enters open sinuses in other parts. 2.An open circulatory system is less efficient at supplying oxygen to all tissues in the body, so it is common in slow-moving animals. ii. The fast-moving cephalopods have a closed circulatory system with several hearts, blood vessels, and capillaries. 1.In a closed circulatory system, blood moves to and from tissues within blood vessels. iii. Many mollusks have blue blood from a copper- containing pigment called hemocyanin; however some with iron- containing hemoglobin have red blood. i. Gas exchange varies depending on the organism and environment they live in; respiratory organs include ctenidia (modified gills), a simple lung, and the mantle. a. Organisms in Phylum Mollusca do not reproduce asexually. b. Most are dioecious, although some are hermaphroditic. c. In some mollusks, the zygote hatches and undergoes two larval stages, trochophore ↑ trochophore and veliger, before becoming a juvenile. i. The free-swimming trochophore larva that ↓ veliger emerges from the egg in many mollusks is remarkably similar to that seen in annelids. ii. Trochophore larvae are minute, translucent, and have a prominent circlet of cilia used for swimming. d. In many mollusk groups (especially gastropods and bivalves) the trochophore stage is followed by a unique larval stage called a veliger. The free-swimming veliger has the beginnings of a foot, shell, and mantle. a. Polyplacophora means ‘many plate bearers’. i. Their shell contains 8 overlapping plates on the dorsal surface. If detached, they can roll up like a pill bugs or armadillo. b. Chitons have a flattened body with convex upper surface. Most range in size from 2-5 cm; but some species can grow to 30 cm. c. All are marine species; they prefer rocky intertidal zones but have been found at 4000 m depths. d. Chitons are fairly sedentary clinging to rocks or other hard surfaces, moving only short distances to feed. Most feed using radula to scrape algae from rocks. a. ‘Gastro’ means stomach, ‘pod’ means foot. i. Gastropods are known as the stomach-footed mollusks because their large muscular foot is positioned under the rest of its body. b.By far this is the largest and most diverse class in Phylum Mollusca, containing over 70,000 living and more than 15,000 fossil species. i. Some of the more unique species include limpets, whelks, conchs, periwinkles, sea hares, and sea butterflies. c. Gastropods can live on land, in freshwater, or in ocean. They are usually sluggish, sedentary animals because most of them have heavy shells and slow locomotion. Many species secrete mucus from the base of their foot forming a slimy path that they can glide along. Just as lipstick contains fish scales, and bird droppings are in spa facials, cosmetics manufacturers are now including snail slime as the latest oddity ingredients. i. Most have a single shell (snails, conchs); Some have no shell at all (slugs, nudibranchs aka sea slugs). ii.The shell, when present, is always of one piece (univalve) and may be coiled or uncoiled. iii.The apex is the oldest and smallest whorl. Whorls become successively larger and spiral around a central axis. iv.The shell may be right handed (dextral) or left handed (sinistral), depending on the direction of coiling.