Faculty of Biological Sciences, University of Leeds

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Evolutionary Developmental Biology Lecture 2

Geological time Phyla then and now, Top of the pops (OVERHEAD)


We saw last time that animals are different. I suggested :

  • First that the meaningful divisions were species and phyla: species is a division that is important to the animal, phylum is a rational division into animals with the same basic way of doing things, and is at least meaningful to us. Organisms that are radically different can go into different kingdoms, plants, fungi, bacteria.
  • Second that the rest of taxonomy, because of the arbitrary nature of the divisions, is a bit like train spotting.


One of the basic ways of classifying organisms is related to their cellularity. In most cases we can just count the nuclei: bacteria don't have a nucleus as such, so they can be placed in a separate file. Organisms that do have nuclei will have either one each, or more than one per individual. These are unicellular versus multicellular organisms, and we can open separate files for these as well. The unicellular organisms, or protistans and the multicellular organisms cut across another border, because some of them have chloroplasts, make their own food from sunlight and are clearly plants while other similar forms seem to have lost their chloroplast, eat food, are not green and are clearly animals.


The earliest eukaryotic organism (i.e. the earliest cell proper) is thought to have been around 1.4 billion years ago. The first multicellular organisms we know about are 800 million years old. This probably doesn't mean that evolving multicellularity took hundreds of millions of years, but that the first metazoa were probably small, soft and didn't make very good fossils.


We will deal here only with animals that are multicellular, because they alone have a development or an embryology.


Multicellular animals can be divided into a number of different sorts or phyla all of which are basically different in morphology and in the way they do things.


Thirty years ago the basis of a good Zoology course was one year on invertebrates and one on vertebrates. This is because there are rather a lot of different sorts of animals. Comparative morphology isn't taught much these days, so the chances are that you won't have met many of them. So I shall try to give you an idea of just some of the different animal types that are around, and their basic bodyplans.


Until just before Christmas 1995 things were fairly easy, as I could have said that there are representatives of 34 phyla, or basic ways of doing things around on this planet, and this number has been constant since the Cambrian, (OVERHEAD) 500 million years ago. Embarrassingly, in December 1995 someone discovered this fellow, (OVERHEAD) an entirely new phylum, not in a cave, not at the bottom of an ocean trench, but on the lips of the common Norway lobster. But apart from this the other 34 phyla have been represented in fossil deposits and as living animals for 500 million years. The odds are that fossil remains of animals like this will be discovered in Cambrian deposits too, now that we know what we are looking for.


Now I don't want to bore you with the 35 different ways animals do things: of the 35 phyla now around the top ten have 2,611,500 species between them, (OVERHEAD) the other 24 have 5062. Lets just sample these top ten, all of which have over 1,000 different species, to give us some idea of animal diversity.


For each of these 10 phyla (OVERHEAD) I will give you a bodyplan or Bauplan in German (German idea - great trainspotters the Germans) and a brief description, plus a couple of illustrations of animals which you have probably heard of.




1. Phylum Poriphera - the sponges (10,000 species)




Bauplan: sessile adult, flagellated cells, internal spicular skeleton, no nervous system.



Sponges are things that used to be around at bathtime b.p (before plastic). A simple sponge (OVERHEAD) is vase shaped with a large excurrent vent at the top and microscopic incurrent pores scattered over its surface. Water is admitted via the pores and leaves via the excurrent vent. There are various specialised cell types (OVERHEAD) which all do one thing rather well. The vase is covered by flattened covering cells which fit together like tiles in a mosaic, forming an epithelium. The large internal cavity is lined by collar cells each with a flagellum. These create a current which passes out via the vent. Large particles are excluded by the incurrent pore size, but small detritus which includes food sticks to the collars and is ingested. Now we know quite a lot about collar cells because there is a group of unicellular organisms, the collar flagellates which resemble them closely (OVERHEAD, back 1). They are even sometimes colonial.


Between the epithelium and the collar cells is a jelly, and in this are amoeboid mesenchyme cells that transport digested food to non feeding cells and store food reserves. They transport waste to the surface where it is carried away by the current of the cilia. They also secrete the skeleton, a series of spicules of calcium carbonate or tough fibres of spongin. Other modified mesenchyme cells sit around incurrent pores and control water inflow by contracting.


So we have a multicellular organism with specialised cells, epithelial cells, feeding cells, transporting cells, skeleton forming cells, muscle cells. The only major group not represented is the nerve cell. Behaviour is apparently a local affair, with individual cells responding as they are stimulated. Sponges can perform one clever trick that most organisms outside cartoons can't (OVERHEAD). If you squeeze them through a piece of silk the cells are split up: after a time they reaggregate in mixed groups and many of these aggregates form new sponges.


Because it has specialised sets of cells arranged in a pattern or a morphology it also must have an embryology (OVERHEAD) which puts the cells in the right place in the pattern. Sponges sometimes also have sexual reproduction. Certain mesenchme cells divide then enlarge to form eggs or sperm. Some sponges are hermaphrodite, but others are unisex. In either case sperm are shed into the excurrent to reach eggs which are retained. The fertilised egg divides, divides again into 4 and more divisions form a hollow ball of ciliated cells one layer thick. The baby sponge is cast into the world at this stage. It swims around, and if not eaten settles down, and becomes a young sponge by a rather complicated manoeuvre.



Sponges are of interest to us because they show, in perhaps its simplest form, the cellular level of organisation, at least as far as morphology goes . The very limited powers of cell co-ordination, and the use of vent as principal orifice suggests that they arose from a different group of protozoa that the main metazoan hoard.




2. Two layers of cells the cnidaria. (10,000 species)


Bauplan: solitary or colonial radial symmetry, tentacles with stinging cells, coelenteron as body cavity, two layered body wall, nerve net.



Another group of animals, the coelenterates or cnidaria are organised rather like sponges, but with a little more sophistication. These are, of course the sea anemones and their relatives the jellyfish and corals.


Lets start with a simple member of this group the freshwater Hydra (OVERHEAD) Superficially this fresh water beastie looks like a sponge, fixed to the substrate (sessile) with cells arranged around a central cavity and an opening at the non-attached end. The likeness is false though, because the large central cavity is a gut, or digestive cavity and the single hole is a mouth (and also, incidentally an anus). Almost all invertebrate and vertebrate animals have a gut which connects to the outside via a single mouth.


The walls of Hydra are made up of two cell layers separated by mesoglea (middle glue) and we see the same sorts of specialised cells as we found in sponges, epithelial, mesenchyme or connective, muscular and reproductive, but here often found sitting together as a tissue, like the muscle fibres in the mesoglia. The important addition here is the nerve cell which is arranged as a nerve net (OVERHEAD) connected by synapses. The presence of the nerve net allows co-ordinated behaviour of the organism as a whole.


Hydras reproduce sexually at certain times of the year (reasons, triggers unknown, but cold and abundant food work in different species) by producing testes and ovaries Some species are hermaphrodite, some have separate males and females. Development again produces a hollow ball of cells - a blastula, then a filled ball - a gastrula. At this stage the embryo usually drops off and may stick to a suitable surface to produce a baby Hydra or sit in the mud until things get better. In any case the outer layer becomes epithelium, a cavity, the gut, appears in the inner mass, a mouth breaks through and tentacles develop.


The marine hydroids (OVERHEAD) are quite clearly related to Hydra, although they are colonial. The most interesting thing about this beast is something called the medusa. This carries the gonads, quite literally, away from the sessile polyps and off to sea Now the medsusa is quite clearly a broken off polyp top Turn a medusa upside down (OVERHEAD) and we have a jellyfish. Jellyfish often have a brief sessile stage in their life (OVERHEAD) which improves on the arrangement we have already seen by making a whole pile of medusae.


If the balance shifts the other way and the medusa stage is eliminated we get back to sea anemones (OVERHEAD), rather upmarket Hydra, which in turn, with a calcified skeleton, produces a coral.




3. A third layer of cells - the Flatworms (Platyhelminhes, 25,000 species)

(OVERHEAD) Bauplan: bilaterally symmetrical, flattened, triploblastic, flame cells, nervous system with brain.


Flatworms are trying out two major innovations.

  • First Planarians (flatworms) have a head end. This is an important development, and worth considering further. So far all the animals we have looked at are either spherically or radially symmetrical (OVERHEAD). Spherically symmetrical animals are based on a sphere, a cut in any plane which includes the centre divides them into two identical halves. A sea anemone is radially symmetrical, about a central axis. A planarian is bilaterally symmetrical. Most animals from now on will be bilaterally symmetrical, although some (like the starfishes) may secondarily loose the symmetry again.
  • Secondly flatworms, (OVERHEAD) and all the animals we shall consider from now on have three layers of cells. The middle layer, or mesoderm, has its origins in the mesenchyme cells we saw in the mesoglia of two layered animals like Hydra: we recognise it as a tissue in its own right rather arbitrarily when it gets large, stops being a collection of wandering cells and starts to contribute to structures in its own right. These structures are the organs, which often contain components from two or all three body layers.


At the head end, which gets there first, are some of the things we would expect to see in any self respecting head. Like eyes (OVERHEAD) and the start of a brain The nerve net has become arranged into a sort of ladder with motorways heading north - south and lesser connections going east - west. The north - south scheme is seen also in the excretory and reproductive systems (OVERHEAD), but the gut is a bit different The gut is in fact Y shaped, with the stem anterior and a cnidaria pharynx stowed between the arms. This means that the mouth is in fact in the middle of the lower surface.


Many of the platyhelminthes are parasitic, including the liver flukes and tapeworms. These have fascinating specialities, but need not concern us here.




4. Wheel animals. Phylum Rotifera (1500species)(OVERHEAD)

Bauplan: unsegmented, bilaterally symmetrical, spherical body with bifurcate foot, anterior wheel organ, jaws in pharynx, cuticle, protonephridia+


Rotifers often don't get much of a mention in Zoology texts. I include them here because they have some interesting features (OVERHEAD). Apart from the ciliated whorly bit which catches food they are a varied group, varying from wormlike to flowerlike, in being attached at one end. But all forms are still basically bilaterally symmetrical. What do they have that flatworms don't? Well a through gut for a start, running from a mouth near the front end to an anus, a new development near the rear, which takes output from digestive, reproductive and excretory systems.



5. The Round worms. Phylum Nematoda (500,000 species)(OVERHEAD)

Bauplan: Cylindrical, unsegmented triploblastic worms, cuticle without cilia, longitudinal muscle fibres, triradiate pharynx, gland cells/canals as excretory system.


Flatworms are flat, roundworms are round.(OVERHEAD) They are a very uniform group, many parasitic, many living in restricted environments which vary from soil to beermats. Some live in dogs, some in humans. This one, quite typical, lives in pigs: more specifically in pigs' intestines.


If we look at the interior arrangements (OVERHEAD) we see the customary three layers, but a rather novel arrangement if you are used to looking at humans. There is no muscle around the gut: instead is a fluid filled cavity, with all the mesodermal muscle around it, creating a positive pressure. This means that food has to be pumped into the gut, which it is by a muscular pharynx, and leave via a muscular sphincter. When a roundworm opens this sphincter a jet may shoot half a metre. The nervous system is a nerve ring around the pharynx and dorsal and ventral nerve cords. One peculiarity is that nerves do not branch to muscles: muscle processes extend to the nerves. Another peculiarity is that nematode sperm, and no others except those of rotifers, are amoeboid rather than flagellate.

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