Protostome The Deuterostomes Echinoderms and Chordates Ch 35 Deuterostomes The SECOND opening of the digestive system (in the gastrula stage) becomes the mouth Deuterostomes Characteristics: Bilaterally symetrical Coelomate Blastopore fate (anus) Second opening in the gastrula becomes the mouth This developmental similarity between echinoderms and chordates is supported by DNA sequence data Phylum Echinodermata (Gk: echin = spiny; derma = skin) Why are Echinoderms considered to be bilaterally symmetrical? 5 Classes: Sea stars, brittle stars, sea urchins, sand dollars, sea cucumbers A thin skin covers an endoskeleton of hard calcareous plates 7,500 species all marine 1
Echinoderm larvae are bilateral Starfish larvae Adults - Pentaradial Sea Urchin larva Sea Cucumber Some species are commercially harvested for human food Echinoderm Features Unique water vascular system A network of hydraulic canals used in: locomotion feeding gas exchange excretion 2
Sea Star anatomy Muscle and nervous tissue, but no cephalization in adults Endoskeleton of hard calcareous plates Complete (but short) digestive system No specialized respiratory, circulatory, or excretory systems Separate male and female individuals External fertilization Tube feet of a sea cucumber Sea Star feeding on a bivalve Everts stomach into the bivalve to digest it INSIDE ITS OWN SHELL!! Sea stars = Keystone predators Keystone species - plays a unique role in the way ecosystems function greater than what their biomass would suggest. Sea stars eat shell fish (urchins, mussels, etc) that most others do not eat If sea urchins and/or mussels (herbivores) take over what will happen? Crown-of-thorns sea star Feed on the living cnidarians that produce coral reefs Population explosions of the crown-of-thorns sea star can severely damage coral reefs Causes of outbreaks appear to be removal of natural predators and increased nutrient levels in the water (both due to human activities) Control efforts include injection of individual starfish with chemical toxins 3
Phylum Chordata This phylum includes the animals that are most familiar to us - and includes humans. Phylum Chordata Divided into 3 subphyla, which include 11 major groups of chordates Some of the 11 groups are defined as Classes by biologists Others of the 11 groups are currently considered to be clades, i.e. formal taxonomic level has not been agreed upon Chordates Fig 35.12 Chordate does NOT = Vertebrate All chordates have four characteristics (at some developmental stage) 1. Pharyngeal gill slits 2. Dorsal hollow nerve cord 3. Notochord 1. Pharyngeal slits Openings to the outside of the body at the pharynx (region just behind the mouth) Allows water that enters the mouth to exit without passing though the gut in invertebrate chordates Modified for gas exchange or other functions in vertebrates 4. Post-anal tail Fig. 34.3 Pharyngeal slits 4
2. Dorsal hollow nerve cord Unique to chordates Other animals have a solid nerve cord that is usually ventrally located Dorsal hollow nerve cord 2. Dorsal hollow nerve cord Nerve cord develops into the central nervous system Brain and spinal cord Ventral solid 3. Notochord A flexible rod located between nerve cord and gut Provides skeletal support for muscle attachment Only remnants of embryonic notochord found in most adult vertebrates Notochord 4. Muscular post-anal tail Skeletal elements (notochord) and muscles Lost during embryonic development in many species Non-chordates have a digestive tract that extends the length of the body Post-anal tail Phylum Chordata Three Subphyla: 1. Cephalochordata 2. Urochordata Subphylum Cephalochordata Called lancelets because of their blade-like shape Chordate characteristics persist into adulthood Globally rare, but some places have high population density (e.g., Tampa Bay, Florida) 3. Vertebrates 5
Lancelet Anatomy Suspension feeders that use their pharyngeal slits to filter out small food particles Typically present buried tail-first in sand Subphylum Urochordata Tunicates ( sea squirts ) Adult tunicates don t look much like a chordate No notochord, nerve cord or tail Filter feeders with incurrent and excurrent siphons Adult tunicates don t look much like a chordate Some tunicate species are colonial But these features are present in the tunicate larval form 6
The Vertebrates Subphylum Vertebrates Chordates with a skull (either bone or cartilage) Includes all of the animals we call vertebrates Finally - something we ve heard of!! All vertebrates have the same characteristics as chordates (at some developmental stage), plus Pronounced cephalization With a skull (cranium) Includes some large and rather obvious animals that are quite familiar to us But remember that vertebrates represent less than 5% of the known animal species on Earth Vertebral column that encloses the nerve cord Replaces the notochord function Endoskeletons that grows with the animal Closed circulatory system Lampreys 7
Hagfish (Clade Myxini) Jawless Lost vertebrae through evolution (ancestors had vertebrae) 8