Acoelomates

THE TRIPLOBLASTIC, ACOELOMATE BODY PLAN

Bilateria

The acoelomates, pseudocoelomates, and eucoelomates

Characteristics

bilateral symmetry

cephalization

triploblastic

acoelomates

Characteristics

no body cavity [fig. 16.11a]; body solid, double-walled sac surrounding digestive cavity;

tissue-organ grade: organs are aggregates of specialized tissues

Taxa

Phylum Platyhelminthes - flatworms

Phylum Nemertea - ribbon (or proboscis) worms

Phylum Gastrotricha - jaw worms, marine, live between sand grains

PHYLUM PLATYHELMINTHES - flatworms; 20,000 spp.

characteristics
1. dorsoventrally flattened
2. unsegmented
3. definite head
4. carnivorous, freeliving and parasites

Learn about flatworms from the University of California Museum of Paleontology website

Learn about flatworms from the University of Michigan Museum of Zoology website

Class Turbellaria - planarians [fig. 19.1]

Characteristics
1. free living, mainly marine, but freshwater and terrestrial
2. locomotion via cilia [Fig. 19.3] and muscular contractions for swimming
3. gastrovascular cavity [Fig. 19.4]
4. pharynx [Fig. 19.5]
5. protonephridia with flame cells [Fig. 19.6]. Protonephridial system in a freshwater turbellarian at top right. image courtesy of BIODIDAC
6. paired nerve cords (ladder) [Fig. 19.7]. Nervous system of Dugesia at bottom right. image courtesy of BIODIDAC
7. cephalization
8. monoecious [Fig. 19.9]; reproduce both sexually and asexually [Fig. 19.8]

Class Monogena [Fig. 19.10]: mainly external parasites of fish; no intermediate host (direct development)
picture and information from Ohio State

Class Trematoda - flukes

Characteristics
1. endoparasites, use suckers to attach to host [fig. 19.11]
2. mouth
3. tegument: [Fig. 19.12]
4. reproductive system is the primary organ system
5. most adults are hermaphroditic (schistosomes are dioecious)
6. indirect life cycle, usually involves two or three hosts [Fig. 19.14]
  1. intermediate (first) host, usually a mollusc)
  2. final (or definitive) host
Examples
1. Clonorchis
  1. Chinese liver fluke [Fig. 19.15]
  2. Pictures and information from Ohio State
2. Schistosoma
  1. blood flukes, cause schistosomiasis
    - pictures and information from Ohio State
    - information from the World Health Organization
  2. other species causes swimmers itch [Box 19.1]

Class Cestoda - tapeworms [fig. 19.17]

Characteristics
1. ribbon-like, highly specialized parasites; 1 mm-25 m long;
2. proglottids
3. scolex
4. no mouth, no digestive system--microvilli on tegument
Examples
1. Beef and pork tapeworms. Pictures and information from Ohio State
  1. Taeniarhynchus saginatus: the beef tapeworm [Fig. 19.18]
  2. Taenia-the pork tapeworm
Life History
1. intermediate host - swine or cattle, larvae encyst in muscles
2. final host - humans:-cysticercosis

PHYLUM NEMERTEA - proboscis worms - 900 species

infaunal marine
simplest organisms possessing a simple circulatory system
complete digestive system with mouth and anus [Fig. 19.20]

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