The Flatworms

 Organ level of organization ◦ Well developed, specialized anatomical structures  Incomplete gastrulation ◦ Gastrovascular cavity with one opening  Triploblastic ◦ True mesodermal tissue filling space between endoderm and ectoderm (note “parenchyma”)  Bilateral Symmetry ◦ 2 symmetrical sides about longitudinal axis  Cephalization ◦ Concentration of sensory apparatus at anterior end

 Acoelomate  Parenchyma derived from mesodermal tissue  Elongated body  No appendages

 Simple integument  Ciliated in free living flatworms, but lacking cilia in parasitic forms  Glandular tissue associated with skin ◦ Mucous glands ◦ “Dual gland” for adhesion and release

 Hydrostatic Skeleton  Parenchymal tissue holds a constant (“static”) volume of water (“hydro”)  Compression/contraction in one direction must necessarily result in expansion/elongation in a different direction  Well developed muscles are present in the body wall ◦ Longitudinal ◦ Circular ◦ Oblique

 Predict the result from the contraction of: ◦ Longitudinal muscle (2) ◦ Circular muscle (1) ◦ Oblique muscle (3)

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 Centralized nervous system ◦ Paired solid, ventral nerve cords ◦ “Ladder” type nervous organization – Transverse nerves connecting longitudinal nerve cords  Concentration of nerve tissue forming cephalic ganglia (simple brain)

 Cephalization ◦ Concentration of sense organs in anterior body region  Sense organs ◦ Photoreceptors (note “ocelli”) ◦ Chemoreceptors (note “auricles”)

 The development of the digestive cavity is incomplete, forming a gastrovascular cavity with a single opening  Digestion is largely extracellular, although some flatworms complete the process in food vacuoles (intracellular)  The class turbellaria are free living and carnivorous, and have well developed gastrovascular cavities  The other classes of flatworms are parasitic, dependent upon the host for digestion

 Mouth located ventrally  Free living turbellaria have an extensible pharynx  The highly branched gastrovascular cavity brings digested food in close proximity to all body tissues  As the gastrovascular cavity has only 1 opening, wastes are excreted through the pharynx or retained

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 The gastrovascular cavity in parasitic forms is diminished (as in this photo) or absent (as in the tapeworms)  When present, the gastrovascular cavity is generally Y shaped

 Osmoregulation is the removal of excess water  Excretion is the removal of metabolic waste (mostly nitrogen compounds like ammonia or urea that accumulate from metabolism of protein) and excess dissolved materials (salts)  The flame cells are concerned with osmoregulation  Metabolic wastes are generally removed by simple diffusion

 “Protonephridia” – primitive excretory tubules driven by Flame Cells.  The “flame” consists of several flagellae that drive fluids through tubules that lead to collecting tubules

 There is considerable variation in the structure of osmoregulation systems in the flatworm classes, from simple excretory pores to a bladder

 Due to the simplicity of the flatworm body form, many are capable of reproducing asexually. In low density populations, asexual reproduction (by budding or fission) may be predominant  Flatworms are monoecious (one reproductive form – AKA hermaphroditic) but generally are unable to self-fertilize  Parasitic forms have complex life cycles, often involving one or more intermediate hosts.

 This flatworm is free living. Its muscular and digestive systems, while not shown above, are well developed  Note the presence of both male and female gonads (Monoecious)

 This liver fluke is parasitic. It depend on its host for locomotion and digestion, allowing it to devote more of its energy and anatomy to reproduction  Note that is also is monoecious. The testes are more pink, and located posteriorly  The ovary is darker and more centrally located

 Tapeworms are parasitic. Their digestive system is absent. Almost their entire anatomy is devoted to reproduction  They have a segmented body, with both testes and ovaries located in each segment

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 The Phylum platyhelminthes is divided into 4 classes ◦ Class Turbellaria – The free living flatworms ◦ Class Trematoda – The digenetic flukes  Digenetic = 2 or more hosts ◦ Class Monogenea – The monogenetic flukes  Monogenetic = 1 host ◦ Class Cestoda – The tapeworms ◦

 The only class of free living flatworms  Ciliated epidermis with mucous secreting rhabdites  Acoelomate  Extensible pharynx  Includes the planaria

 Parasitic flukes  Syncytial tegument (multinucleate skin) without cilia  Y shaped gastrovascular cavity  Oral sucker for obtaining food and ventral sucker for attachment  No hooks for attachment to host  Digenetic – 2 or more hosts ◦ Intermediate host – usually an invertebrate, frequently a mollusc ◦ Definitive host – a vertebrate  Includes fasciola, schistosoma and clonorchis

 Parasitic flukes  Syncytial tegument (multinucleate skin) without cilia  Free living, ciliated larvae  Y shaped gastrovascular cavity  A single oral sucker  A series of posterior hooks (opisthaptor) for attachment to the host  Monogenetic – parasitic on a single host, usually a fish, amphibian or reptile

 Tapeworms, parasitic to the digestive tracts of vertebrates  Syncytial tegument without cilia  The tegument is covered with fingerlike extensions to increase the surface area for food absorption  Gastrovascular cavity is absent  Anterior structure (scolex) with hooks/suckers for attachment to the host  Body is formed into segments called proglottids. Each proglottid has testes and ovaries. Self- fertilization is possible  Gravid proglottids from feces of infected animals may be consumed by a new host, or may be spread by consuming larvae in the flesh of an infected animal

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