An Online Introduction to the Biology of Animals and Plants





Key Concepts




Section 3

Chapter 6

Segmented Worms / Annelids






Segmentation - A Body Plan to Build Upon




Each new group seems to bring some huge evolutionary leap that will form a new foundation for later groups to build upon, and this group is no exception.  The new feature this time shows up in a few mollusks, such as the chitons, but its unclear what effect it had on most of the mollusks history.  There seems a good chance that this chapters group has some mollusks on its family tree.

The feature is segmentation (technically called metamerism, probably to avoid confusion among all the different types of "segments" in animals).   The basic approach is to come up with a basic body "compartment," like cars on a train, and build up a body by adding more.  It is a quick and simple way to add size, and segmented worms, also known as annelids, are the prototypes.  In animals that stay fully segmented, it helps make movement more efficient, but in many groups it shows up mostly as a developmental pattern - our phylum, the Chordates, are considered segmented, although the patterns are clearer in embryos than in adults.

Somewhat like a classic train with its engine and caboose, segmented animals use their repeating segments between specialized front and back compartments.  There is a typical head, with a mouth and sensory organs and processors (segmented worms show
cephalization and the connected bilateral symmetry), and a back end, with the exit of the digestive system and sensory organs to detect things sneaking up from behind.  The digestive system usually is not segmented, just running down the sequence of segments in its classic disassembly-line configuration.

A typical
segment has similar set-ups in blood vessels (segmented worms circulate their blood in tubes, a system called closed circulation);   nerves (each segment connects to the next along the typical invertebrate central nervous system, two solid cords running along the ventral [belly] side), usually with processing ganglia in each segment;  muscles, running both lengthwise (longitudinal) and around (circular);  a doughnut-shaped, fluid filled chamber that acts as a hydrostatic skeletonexcretory structures;  and hard bristles called setae sticking out and often supporting paddlelike structures called parapodsThis sequential pattern of repeating architecture is called serial homology, and is the reason why human arms and humans have the same basic bone structure.

Segmentation is a very efficient way to build an animal, but being extremely segmented once the pattern is set is apparently unnecessary.  Even in the segmented worms, there are whole groups where the segmentation in adults is mostly on the surface.  Some segmented animals in other groups, like the centipedes, remain very much segmented throughout their lives;  others, like flies or humans, use a segmented pattern to lay out their embryos and then modify it until it almost disappears.

Some sources claim that the segmented muscle layout and articulations between each segment lead to more efficient movement, and that may have been part of the advantage as segmentation first evolved.  It must be a minor advantage, however, since many efficiently-moving animals are among those that largely lose their segmentation during development.






Segmented Worm Subgroups



This phylum has three major subgroups:

 - The polychaete worms are almost totally marine.  Most are small, but they occupy all sorts of niches in the environment in and near the bottom.  They include swimmers, preying upon the small and themselves being preyed upon, the crawlers, scavenging across the sediments, the sitters, filter feeding, and the burrowers, consuming sediments the way that earthworms eat dirt.

 - The oligochaete worms include most fresh water segmented worms, similar to their marine cousins but more often outfitted with setae but no parapods.  Also in this group are the earthworms, which burrow through moist soils and extract nutrients from dirt, aerating the soil with their burrows and helping nutrient recycling with their wastes.

 - The leeches are less segmented than the other types, although the basic body plan of the adult almost always starts with 34 segments between front and back (this is common in other segmented groups, where a set number of segments arises in embryos as the base architecture).  They have broader, flatter bodies than the other segmented worms, often used for swimming, and suckers at each end used while crawling inchworm-style.  They live on blood, but are just as likely to take it while preying on smaller animals as to suck it from larger ones.  Only some leeches are obligate (meaning they have to live a certain way) parasites;  most are facultative (meaning they do it when they can but dont have to) parasites.

Leeches production of powerful anti-clotting agents have led to their medical use during reattachment of severed limbs and fingers - their presence at the attachment point helps to keep blood flow up.






Informational Links



An information page on oligochaete worms.

Lots of photographs of polychaete worms.

One of the oddest festivals in the world, involving worm sex.






Click on term to go to it in the text.
Terms are in the order they appear.



Segmentation / Metamerism  
Segment Features  
Closed Circulation  
Central Nerve Cord in Invertebrates  
Longitudinal Muscles  
Circular Muscles 
Hydrostatic Skeleton  
Serial Homology  
Segmented Worm Subgroups  
Polychaete Worms 
Oligochaete Worms  





Go On to Next Chapter - Arthropods






Online Introduction to the Biology of Animals and Plants.

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