Zoology 250 Lecture 14/15
ASCHELMINTHES (=PSEUDOCOELOMATES)
(Nematodes, rotifers, tardigrades & relatives)
- 1) A grouping of convenience whose origins and membership remain controversial; features shared by most (or all*) phyla are:
- a) triploblastic organization* (extensive mesoderm) with a complete gut (8 of 9 phyla)
- b) small size (most <5mm) associated with: eutely (fixed cell number in many sp.), 'pseudocoelom' (7 of 9 phyla); lack blood-vascular system*, protonephridial excretory system (7 of 9 phyla)
- c) determinate cleavage* with mesoderm from blastopore lip*
- d) retractile or eversible spiny head end (6 of 9 phyla)
- e) an acellular cuticle that must be molted (6 of 9 phyla)
- 2) A pseudocoelom differs little from a hemocoel (no epithelial lining):
- a) it is not a persistent blastocoel, but develops just like a hemocoel in other protostomes (separation of connective tissue), and the gut is often free of mesodermal tissue; it is not lined by epithelium
- b) 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
Phylum NEMATODA
- 4) PHYLUM NEMATODA (round worms)
- a) may be most numerous of metazoans & 2nd most diverse phylum
- b) most are free living, but many are also parasitic
- c) cylindrical or spindle-shaped bodies surrounded by tough cuticle
- d) large forms have an extensive pseudocoelom
- e) body wall muscles are exclusively longitudinal
- f) 4 nerve cords: dorsal & ventral (locomotion), 2 lateral (sensory)
Phylum ROTIFERA
- 5) PHYLUM ROTIFERA ("wheel bearer" worms)
- a) among the smallest of metazoans, widespread in freshwater
- b) huge variety of body forms & life cycles; often have a 3-part body (head, trunk, foot) with a ciliated crown (=corona)
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(revised Jan. 14, 1999)