Hydrostatics--Continued

Nematodes

Body construction

• Cuticle--non-living connective tissue fibers, laid down in many different directions. Analogous to the multiple plies on modern tires. In many groups, must be molted for growth to occur.
• Coelom--pseudocoelomate construction. Gut has no mesoderm covering, but outer body wall has mesodermal cells. Those cells have a basal muscle layer and vacuolar inner portion with fibers that connect to dorsal or ventral nerve cords.

Functionality

• Longitudinal fibers contract against the fluid in the pseudocoel
• As a result, the body oscillates in a lateral fashion with a wave passing from the anterior to the posterior end.
• Very large pressures are built up in the pseudocoel. These tend to compress the gut cavity.

Typical example

• Ascaris

Annelids

Oligochaetes

Body Construction

• Very thin cuticle, body wall of longitudinal and circular muscles
• Coelom is segmented and divided into left and right components
• Each segment bears two pairs of setae

Functionality

• Wave of contraction/relaxation passes anterior/posteriorly.
• Circular muscle contract makes each segment long and slender, longitudinal muscle contraction makes each segment short and fat.
• When short and fat, setae touch environment, when long and slender setae do not touch the environment.
• Wave of contraction is symmetric and called a peristalsic wave.

Polychates

Body Construction

• Septa between segment and between right and left halves are not always developed, so coelomic fluid can move between all areas.
• Body wall has outgrowths called parapodia which contain many setae and bristles (What is the difference between a setae and a bristle?).
• Longitudinal muscle more or less in two vental bands that run anterior/posteriorly.

Functionality

• Assymetric wave of contraction passes from the anterior end posteriorly.
• This produces a pattern of contraction that looks like a sine wave.
• When the wave is on the inside, the parapodia come in contact with the environment and the setae/bristles act as fulcra points, that the body is contraction against. On the outer edge of the sine wave the parapodia are lifted off the substrate and moved forward.

Molluscs

Gastropods

Body Constructon

• Foot is like an elongated pancake. Some muscle fibers run longitudinally, others run vertically. Origin/insertion of vertical fibers is not above each other. Thus these fibers run at a slight angle.
• Foot itself acts as a hydrostatic skeleton as the tissue is mostly water which the two sets of fibers can contract against.

Functionality

• Waver of contraction passes anterior to poserior. As it reaches vertical fibers in one location, those fibers retract the sole of the foot from the substrate. When the next phase of the wave reaches that location the foot is again appressed to the substrate and that part of the sole moves forward.
• Sometimes the wave is out of phase on the right and left sides of the foot, so the sole of the foot sort of shuffles along.