Basic groups 5: Animals II

In this post, we will cover a two more worm phyla. In subsequent entries, we will see more miscellaneous ‘lower’ invertebrates, before moving on to the ‘higher’ invertebrates, and finally to the chordates.  

Annelida, the ringworms, is a group of advanced worms (but, worms being fairly primitive, they are still relatively simple animals). They are coelomate triploblasts with bilateral symmetry (if these terms are unfamiliar, please see Part 4). The coelom (body cavity) is filled with fluid, which acts as a skeleton: as muscles work against the incompressible fluid, the body changes shape, creating movement in relation to the environment. This type of skeleton is termed a hydrostatic skeleton, and is present in several animal phyla. The specific movements vary. The video below shows how a typical annelid (and earthworm) moves forward.

Locomotion of an earthworm. By compressing the sides of the coelom (using muscles circulating across the long axis), it expands in length; by relaxing the side pressure (and contraction of muscles running long the length of the worm), the worm shortens; the front anchors into the substrate between lengthening and shortening, which makes the annelid’s net movement forward.

This is indeed a very primitive way of moving, but I personally find it fascinating in its simplicity and apparent ingenuity.

The undulating contraction series that occurs during locomotion (peristaltic movement, in formal jargon) is made possible by the segmented body. The worm is divided into multiple segments, which contain repeated sets of certain organs and muscles. The circular muscles that create the contraction wave are repeated in each segment, and so are the muscles running along the long axis, which help pulling the animal together. The excretory organs, called nephridia (which are not much like our kidneys), are also repeated, in pairs. Extensions of the semi-centralised nervous system also spread out in each segment.  The blood flow of the circulatory system (internal transport of nutrients and gases, such as oxygen and carbon dioxide) is also organised with regard to the segments, while being connected throughout the animal.

Annelids also show a greater degree of cephalisation, compared to the more primitive platyhelminths (flatworms). In the front end, there is a concentration of nerve cells, a set of five ‘hearts’ (rings that function as pumps to make the blood flow through the vessels), a pharynx and an oesophagus, and specialised gut sections: a crop (for brief storage) and a gizzard (muscular section that can grind food material before passing it on to the intestines).

The leeches (Hirudinea) are a bit of an exception: their heads are simplified and modified into a blood-sucking device we are familiar with. They can use their suckers in front and back (the hind sucker is always larger) to move on a hard substrate, or swim around in water.

Unlike the poriferans, cnidarians and platyhelminths, the annelids are too complex to regenerate with such ease, although some are capable of recovering lost parts to some extent.

An annelid (member of the group Polychaeta), perhaps less familiar than the typical, 
bristle-less earthworm (Oligochaeta). Image from http://www.mediahex.com/Polychaete

A leech. Image from http://bio1151.nicerweb.com/Locked/media/ch33/leech.html

Nematoda is another group of worms, and, like the platyhelminths, they are mostly parasitic. They are pseudocoelomate, an intermediate between the acoelomate platyhelminths and the coelomate annelids (although they belong to different evolutionary groups). The nematodes are triploblastic, with a bilateral symmetry, but they are not segmented.

Perhaps surprisingly, the nematodes are actually more closely related to arthropods (insects, spiders, crustaceans, etc.) than to any worm phylum. (NB: once we have gone through the animal phyla, I will spend some time explaining how they are related, and hopefully it will create a clear picture of how the animals have evolved.) This is because they both have a hard external cuticle, a protective, multi-layered structure, composed primarily of collagen (a protein, common in connective tissues of many animals) in nematodes, and chitin (a sugar) in arthropods. The cuticle is smooth, and the nematodes have no obvious head, so there are basically no external features that characterise them – but, perhaps the lack of features itself is useful for recognising them!

A nematode. Image from http://www.flickr.com/photos/slider5/418135144/

The cuticle is rigid, and cannot grow together with the rest of the nematode, so it needs to moult – i.e. shed its cuticle and grow a new one that fits – just like arthropods; some snakes are also known to shed their skin.

This cuticle is layered in a way that makes it bendy, although inelastic, enabling the nematode to move. The nematodes only have muscles that run along the long axis, so they can only move by wriggling… *hrrmm* sorry, I should say waves of undulatory movement. A curious thing about nematodes is that they wriggle up to down, but swim on their side, so it looks like they wriggle sideways, like snakes.

Movement is possible thanks to the hydrostatic skeleton. This, coupled with the hard external cuticle, means that the nematode has high internal pressure. This has two important conesquences: first, the nematode requires a muscular pharynx in order to swallow food, because the intestines are under such pressure; second, if the cuticle breaks by accident, the nematode more or less explodes and dies. Therefore, the Nematoda does not possess any regenerative abilities, since damage basically leads to instant death.