Zoology 250 Lecture 14
ASCHELMINTHES (=PSEUDOCOELOMATES)
(Nematodes, rotifers, tardigrades & relatives)
(Aschelminthes study images)
- 1) A grouping of convenience whose origins and membership remain controversial; features shared by most (or all*) of 9 phyla are:
a) triploblastic organization (extensive mesoderm)*
b) complete gut (8 of 9 phyla)
c) small size (most < 5 mm) associated with:
- eutely (fixed cell number in many spp.)
- 'pseudocoelom' (7 of 9 phyla)
- lack blood-vascular system (have a pseudocoelom)*
- protonephridial excretory system (7 of 9 phyla)
d) determinate cleavage* with mesoderm from blastopore lip*
e) retractile or eversible spiny head end (6 of 9 phyla)
f) an acellular cuticle that must be molted if thick (6 of 9 phyla)
- 2) Larger aschelminthes posses a fluid-filled pseudocoelom
a) it differs little from a hemocoel anatomically (partial or no epithelium separating fluid from tissues)
b) it is not a persistent blastocoel
- it develops just like a hemocoel in other protostomes (separation of connective tissue)
- the gut is often free of mesodermal tissue
- it is not lined by epithelium
c) it is not associated with a pumping heart, and thus functions mainly as a hydrostatic skeleton
- 3) Most have a collagen or chitin cuticle that has many functions:
a) as a skeleton it may antagonize longitudinal muscles
b) it lines the pharynx in many forms and may give rise to teeth or spines important in locomotion or feeding
c) in some groups it is quite tough and must be molted to grow
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(revised Mar. 28, 2000)