Chapter 33 Protostome Animals - insects the phylum Arthropoda include the insects, crusraceans and myriapods and make up 40% of the total mass of organisms present 33.1 An Overview of Protostome Evolution - phylogenetic studies supported the hypothesis that protostomes are a monophyletic group. This result means that the protostome developmental sequence arose just once. 2 monophyletic groups of protostomes that exist within the protostomes are the Lophotrochozoa and Ecdysozoa, they were identified by DNA and RNA sequences. What is a Lophotrochozoan? -14 phyla include mollusks, annelids and flatworms. - grow by incremental additions to their body, the groups name was inspired by morphological traits that are found in some, but not all of the phyla in the lineage 1) a feeding structure called lophophore which is found in three phyla nad 2) a type of larva called a trochophore, which is common to many of the phyla in the lineage. - a lophophore (tuft-bearer) is a specialized structure that rings the mouth and functions in suspension feeding. Consists of tentacles that have ciliated cells along their surface. The beating of the cilia generates a water current that sweeps protist and other algae into the region above the mouth opening and ingested. - found in bryozoans (moss animals), brachiopods (lamp shells) and phoronids (horseshoe worms) - a trochophore (wheel bearer) is a type of larva common to marine moluscs, annelids that live in the ocean and others. The larva has a ring of cilia around its middle that help swimming. Can produce water currents that sweep food particles into the mouth, primariraly feed on yolk provided by the mother. What is an Ecdysozoan? - the main contrast between 2 are the method of growth. Instead of growing by steady and incremental additions to the body, ecdysozoans grow by molting-shedding an exoskeleton.
- An individual sheds its outer layer called a cuticle or an exoskeleton. The hypothesis behind these structures is based on protection from predators. When an old exoskeleton is shed another one develops in a couple of hours, during this time individuals hide to escape predation 33.2 Themes in the Diversification of Protostomes - 22 phyla with distinctive body plans or specialized mouthparts How do Body Plans Vary among Phyla? - all are triploblastic and bilaterally symmetric and all undergo embryonic development in a similar way. - Radical changes occurred in coelom formation as protostomes diversified. - After the coelom evolved there was a reversion to an acoelomate body plan-meaning the lack of body cavity in Platyhelminthes. - The body cavity pseudocoelom which forms from an opening that originates between the endoderm and mesoderm layers of embryos, arose independently in the protostome phylum Rotifera (rotifers) kolovertku, and the ecdysozoans. - Another change was a reduction of the coelom. - Protostome phyla have wormlike bodies with a basic tube-within-a-tube design. The outside tube is the skin which is derived from endoderm, muscles and organs are derived from mesoderm are located between the 2 tubes. - In the wormlike phyla the coelom is well developed and functions as a hydrostatic skeleton that is the basis of movement. - In Arthropoda and the Mollusca, the coelom is reduced, it is a vestigial trait. They don t need one, they have other structures to sustain this function - Coelom functions: providing space for fluid to circulate among organs and providing a hydrostatic skeleton for movement. The Arthropod Body Plan - arthropods have segmented bodies that are organized into prominent regions called tagma. In the grasshopper these regions are the head, thorax (GRYDNA KLITKA), and abdomen. In crustaceans the exoskeleton is strengthened by the addition of CaCO3. - Arthropod locomotion is based on muscles that apply force agains the exoskeleton to move legs or wings.
- The evolution of limbs(konechnost) made a hydrostatic skeleton unnecessary. - Arthropods have a spacious body cavity called hemocoel (blood hollow) that provides space for internal organs and circulation of fluid. In caterpillars the hemocoel functions also as hydrostatic skeleton. The Molluscan Body Plan - based on three components: 1) the foot, a large muscle located at the base of the animal and usually used in movement 2) the visceral mass the region containing most of the main internal organs and external gill and 3) the mantle a tissue layer that covers the visceral mass and that in many species secretes a shell made of calcium carbonate. - Some mollusks are single shelled, some have 2, 8 or not at all - The coelom s function is replaced by the visceral mass and by the muscular foot. - Don t have limbs Variation Among Body Plans of the Wormlike Phyla - similar in most protostome phyla that have wormlike bodies. These lineages are distinguished by specialized mouthparts or feeding structures. - Echiurans (spoon worms) burrow into marine mud and suspension feed using an extended structure called a proboscis which forms a gutter leading to the mouth. Cells in the gutter secrete mucus, which is sticky and captures pieces of detritus. Then this whole thing is swept to the mouth by cilia on cells in the gutter. - Priapulids (penis worms) burrow into the substrate but act as sit and wait predators. When pray approaches the priapulid everts its tothed, cuticle lined throat meaning that it turns its throat inside out, grabs the prey item and retracts the structure to take in the food. - The ribbon worms they have a proboscis that can extend or retract. Spear small animals with their proboscis or wrap the extended proboscis around prey, then pull into the mouth. The Water-to-Land Translation - made the transition from water to terrestrial environments. Occurred multiple times as protostomes diversified - The ability to live in terrestrial environments evolved independently in arthropods (at least twice), mollusks, roundworms and annelids - It was either for protostomes to move to terrestrial environments. New adaptations allowed protostomes to 1) exchange gases and 2) avoid drying out.
- Water to land transition is important because they open up entirely new habitats and new types of resources to exploit. How Do Protostomes Feed, Move, and Reproduce? - once the wormlike, arthropod and mollusk body plans had evolved, subsewuent diversification was largely driven by adaptation that allowed protostomes to feed, move or reproduce in novel ways. Adaptations for Feeding - include suspension, deposit, liquid and food-mass feeders. Feed on plants, algae or other animals. - Arthropods have the same body plan but their mouthparts are the most diverse and allow them to pierce, suck, grind, bite, mop, chew, engulf, cut or mash - The crustaceans use their legs to sweep food toward their mouths as they suspension feed; certain insects spiders and mollusks use their appendages to capture prey - Metamorphosis is extremely common in protostomes and results in larvae and adults that live in different habitats and have different overall morphology and mouthparts. Adaptations for Moving - devends on variation in 2 features 1) the presence or absence of limbs and 2) the type of skeleton that is peresnt, - worms and other insect larvae don t have limbs move with their exoskeleton. - Although insect larvae have a highly reduced coelom, they do have an enclosed fluidfilled body cavity that functions as a hydrostatic skeleton - The evolution of limbs allowed for running. - The insect wing is one of the most important adaptations. 2/3 multicellular species are winged isects. The earliest insects had 2 pairs of wings. 4 winged, only use only 2 in flying. In flies, they fly with 1 pair of wings, have small winglike structures called halteres that provide stability - In mollusks, waves of muscle contractions sweep down the length of the larde, muscular foot. Cells secrete mucus which reduces friction and increases the efficiency in gliding - When the cavity surrounded by the mantle fills with water and the mantle muscles contruct a stream of water is forced out of a tube called siphon.
Adaptations in Reproduction - asexual reproduction by splitting the body lengthwise or by fragmenting the body is common in many wormlike phyla - crustacean and insect species reproduce asexually via parthenogenesis (virgin origin) the production of unfertilized eggs that develop into offspring. - Sexual reproduction is often based on external fertilization and is predominant. - 2 important reproductive innovations occurred during protostome diversification 1) the evolution of metamorphosis and 2) an egg that would not dry out on land. - Metamorphosis is common in marine protostomes, hypothesized that its an adaptation that allows larvae to disperse to new habitats by floating or swimming in the plankton. Common in terrestrial, hypothesized that an adaptation reduces competition for food between juveniles and adults - Insects eggs have a thick membrane that keeps moisture in and the eggs of snails have a thin CaCO3 shell that helps retain water. - Desiccation-resistant eggs evolved repeatedly in populations that made the transition to life on land. - The membrane bound egg of insects evolved in the terrestrial vertebrates.