General Zoology (Biol 111H)
|The Triploblastic, Acoelomate Body Plan|
The Triploblastic, Acoelomate Animals
Several minor phyla (we will introduce but not study in detail).
Phylum: Mesozoa (middle-animals)
- marine, endoparasites (mollusk), no organs, ciliated, sexes separate, approximately 100 species .
Phylum: Placozoa (flattened-animals) General Characteristics:
- all marine.
- single species (Tricoplax adherans).
- Consist of two epithelial layers with fiber cells (used in locomotion).
- rediscovered in the Red sea in 1960.
Flatworms that we will study in detail are Triploblastic, having an Acoelomate Body and include the:
Phylum: Platyhelminthes Class: Turbellaria (free-living) Class: Monogenea Class: Trematoda (flukes) Class: Cestoidea (tapeworms) Phylum: Nemertea
Welcome to the Flatworms
General Characteristics: - Approximately 300 species known world wide. - Represent the free living "flatworms." - Aquatic (both freshwater and marine); someterrestrial . - Some with color (pigments). -Triploblastic; dorso-ventrally compressed with anterior / post ends. - Acoelomate; no segmentation. - No larvaestages. - Incomplete digestive system. - Cephalized anterior ganglia. - Excretory / Osmoregulatory System is based on Protonephrida. - Hermaphroditic (monoecious). - Animals with primitive organ systems. - Locomotion by cilia and undulations - Digestion System and Nutrition: 1. Blind - pouch System, branched to increase surface area. 2. Pharynx highly developed. 3. Mostly carnivorous or scavengers, some herbivorous. 4. Some use "chemosensors" - food detection. 5. Digestion is extracellular. - Muscle System: 1. circular, and 2. longitudinal. - Nervous System, generally well developed with posterior fibers and anterior ganglia. - Sensory, some with eyes and statocysts. - No circulatory or skeletal Systems. - Highly developed reproductive System. - No respiratory System - by diffusion only
A Generalized Cross-section of a Turbellarian For example:
A Side -view of Turbellarian Digestive System
¤ some marine forms lack a digestive cavity. ¤ blind cavity, highly branched to simple pouch. ¤ muscular pharynx that functions as an ingestive organ, being ciliated and typically "housed" in pharyngeal sheath.
Pharyngeal gland proteolytic enzymes (catalyze protein digestion).
Mostly carnivorous, some scavengers and others herbivorous.
Food detected by chemosensory cells.
Turbellarian Nervous System
Nervous System - Three types giving rise to 3 separate orders of the Class Turbellaria. 1. Order: Acoela - most primitive ¤ a subepidermal nerve plexus. ¤ similar to nerve-net of cnidarians. ¤ anterior statocyst detects position. 2. Order: Poycladida ¤ similar to Acoela but with a more centralized nerve-net and central ganglia. 3. Order: Tricladida ¤ also a subepidermal nerve - net but pair of lateral nerve cords ¤ nerve cords connected by commissures forming a ladder - like system. ¤ nerve cords connect anteriorly to ganglia.
- Generalized Nervous System of Three Orders of Turbellaria
Order: Acoela Polycladida Tricladida - Central anterior ganglia function as a primitive brain. - Neurons organized into 1. Sensory - nerve fibers leading to the "brain" 2. Motor - nerve fibers leading away from the "brain" 3. Association - nerve fibers that connect or integrate with other neurons - These attributes in neuron inter-relationship (integration) and central control by the primitive brain
demonstrate major advances in the Central Nervous System (CNS). - Sensory 1. Tactal - located in auricles of sensory lobes, detectind pressure (mechanoveceptor)
of water currents. 2. Chemosensory - dense clusters of sensory cells in auricles, aid in locating food and each other.
3. Ocelli - eye spots of dense cell masses located anteriorly and lateral to central ganglia ;
serve as photoreceptors allowing response to light (most worms are negatively phototropic)
have black photosensitive pigment; Ocelli cell receptors have neuron connections to
The Turbellarian Reproductive System Reproductive System - Arises from mesodermal tissue. - Mostly hermaphroditic (monoecious) - Sexual 1. Well developed male / female with gonads, ducts , bursa that receives
sperm for storage, and copulatory structure (penis). 2. Usually involves cross-fertilization when sperm-mass is exchanged. - Asexual 1. Transverse fission to produce two zooids.(Gk.living animal) 2. Development is direct. 3. Produce cocoon (summer & fall) or have "Muller's" larva (ciliated with arms for feeding/swimming);
may bePaedomorphic (Gk. pais = child) being sexually mature as a larva (neotony). - Cross-sections of Turbellarian Reproductive System
mostly hermaphroditic (monoecious)
Ectolecithal = yolk outside ova
Endolecithal = yolk contained w/i contained cytoplasm
- Copulation in Turbellarian Worms - Hypodermic Impregnation in Turbellarian Worms
General Characteristics: - mostly ectoparasites on fish, or sometimes turtles, frogs, crayfish. - fertilized egg (zygote) has "thread" that sticks to the gill of a fish host.
- matures to a ciliated larva known as an oncomiracidium that swims to another fish host
to mature into an adult worm.
Class: Trematoda (flukes)
(Gk. trematodes = pierced) Many of considerable economic importance. General Characteristics: - No cilia in adult stage. - dorsoventrically compressed. - Muscle System - both circular and longitudinal. - Excretory System - flame - cell Protonephridial system. - No circulatory System. - Respiration via diffusion. - Nervous System with a pair of anterior ganglia and lateral fibers. - Digestive System - dead-ended pouch.
¤ has anterior and ventral suckers used as attachment organs (used also in classification of flukes
- Reproductive system - well developed, monoecious and hermaphroditic. - No skeletal system (not found in any Platyhelminthes)
Subclass: Aspidogastrea General Characteristics:
- all parasitic flukes to mollusk. - all with large "hold - fast " organs, known as the opisthaptor. - no oral sucker. - complex life cycle. - in some, a vertebrate may serve as thefinal host becoming infected when eating a mollusk
that carry immature aspidogastreans.
Phylum: Platyhelminthes Class: Trematoda Subclass: Digenea
(Gk. di = two + genea = generations )
- Adults endoparasitic to vertebrates. - at least 2 different life cycle forms in 2 or more hosts. - parasites have an oral sucker surrounding a mouth and
a ventral sucker known as the acetabulum. - complex life cycle involving a molluscan (snail) intermediate host and typically 2 or more final
hosts (definitive) of various warm blooded(endothermic) animals, including man.
- Tapeworms, over 3500 species known. - Parasitic, from few mm to 15m in lengths. - No digestive tract or system. - Have great reproductive potential. - Are of considerable economic importance. - Two subclasses: 1. Subclass: Cestodaria ¤ Body not subdivided into individual proglottids.
¤ Larva found in crustaceans, and adult fish. 2. Subclass: Eucestoda ¤ True tapeworms with scolex, neck, strobila with strobila of many proglottids.
¤ Adult tapeworms found in the G.I. tract of vertebrates.
General Characteristics: - usually marine - small phylum of 600 to 1000 species - dorso-ventrally compressed - similar to Turbellarian but show some advancements in that they - have rudimentary circulatory systems with hemoglobin to transport O2. - reach lengths of 30 meters. - Have external cilia for locomotion. - Some with "eyes" and show bilateral symmetry - Muscle system highly developed with circular and longitudinal muscles. - complete Digestive system - Sexes separate, fertilization external. - Respiration through body wall. - Excretory System is well developed, protonephridia - Nervous System with pair of well developed ganglia, dorsal to mouth connecting with nerve fibers forming nerve trunk. - Circulatory system consists of: 1. Three longitudinal vessels. 2. Contains blood with hemoglobin. 3. Forming a "closed" system, no pump. - Reproductive System - Acoelomate and Triploblastic - No Skeletal System.