Faculty of Biological Sciences, University of Leeds

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Evolutionary and developmental Biology Lecture 3.

The top ten phyla (continued )(OVERHEAD)

 

 

6. Slugs and snails the Phylum Mollusca (100,000 species) (OVERHEAD)

Bauplan: body of head, foot and mantle which often secretes shell, complex alimentary canal, nervous system of circumoesophageal ring, pedal cords and visceral loops, gills, coeloem may be reduced, blood system with propulsive heart, arterial and venous system, haemocoele.

 

Quite often a successful group of animals takes off in lots of different directions: this happened to the molluscs, the second largest phylum. A typical mollusc, that is an unspecialised one, is the rather unfamiliar Chiton (OVERHEAD). These are sluggish seashore grazers that clamp down rather like a limpet when disturbed under a row of chitinous plates. Once inside we can see the characteristic mollusc feeding system, a rasp or radula (OVERHEAD) that is protruded through the mouth to file away at food. Behind the radula the gut is conventional with a stomach and digestive glands opening into it. There is a ventral, paired nerve chord and a nervous ring around the pharynx. Most of the ventral side of the animal is made up of a muscular foot.

 

The molluscs that you are more familiar with include the edible ones, snails and slugs, the scallops whelks and clams. All these have had an exiting embryology when their larvae (OVERHEAD) get into a twist which results in the gut crossing over itself. This produces, with a modified shell the bivalve clam (OVERHEAD), and with some more twists, the snail. But the molluscs haven't finished yet: the elegant cuttlefish, squid, octopus and Nautilus are all molluscs (OVERHEAD).

 

 

 

 

7. Segmented worms. Phylum Annelida (14,000 species)

Bauplan: bilaterally symmetrical, segmented, hydrostatic skeleton, cuticle, chaetae, triploblastic external and circular and longitudinal internal muscles, preoral ganglia, paired ventral nerve cords, nephridia, coelomoducts, closed tubular circulatory system. The other novel feature is that within the mesoderm of annelids is a fluid filled body cavity.

 

More than half the phyla in the animal kingdom are worms, bilaterally symmetrical animals longer than they are wide. But mention worms to most people and they think of earthworms (OVERHEAD), annelids.

 

The most notable feature of earthworms and their relatives is the ringing of the body, which is internal as well as external, and involves most body structures: adjacent segments are separated by septa or partitions. Annelidia means ringed. This repetition of structures in such a regular way is new, although it was hinted at in flatworms (OVERHEAD). If we look at a cross section of an annelid (OVERHEAD) we can see that there is a fluid filled cavity in each segment. But this is not like the Ascaris one: it is in the middle of the mesoderm, so there is a layer of mesoderm around the outside of the gut, another around the inside of the ectoderm and a coeloem on each side. In the midline the 2 layers of mesoderm form mesenteries, and in the mesenteries run blood vessels, nerve cord and gut. Ciliated ducts allow waste and eggs/sperm to leave. This beast is thus built like a ship, with watertight compartments, and longitudinal and circular muscle layers around the outside act on the fluid filled bag when burrowing. Exactly the same set-up allows independent peristalsis in the gut. In many annelids, including the earthworm, the ventral mesentery is lost during embryonic development

 

The segmental nature of the beast means that many organ systems are also segmental: the nervous system (OVERHEAD) has a ventral nerve cord with segmental ganglia and segmental nerves. There is a closed circulatory system with a dorsal and ventral main vessel, again with segmental links, 5 pairs of which are modified to form muscular hearts. in fact earthworms are rather specialised annelids: most common are the marine bristle worms.

 

 

8. Lobsters, spiders and flies Phylum Arthropoda (>1,000,000 species) (OVERHEAD)

Bauplan: triploblastic, bilaterally symmetrical, metameric segmentation, paired, jointed appendages, one or more pairs of jaws, chitinous exoskeleton, tubular gut, striated muscles, ventral nerve cord of segmental ganglia, cilia in sense organs, reduced coelom, haemocoel, heart.

 

Another segmented group. Paradoxically the greatest advantage of dividing the body into a series of identical segments is that different segments can be modified to do different things: another level of organisation, above tissue and organ level. Annelids only do this on a small scale: arthropods do it extensively.

 

Over three quarters of animals alive today are arthropods - crustaceans like lobsters and crabs, arachnids like spiders and scorpions or insects. What do these have in common? Apart from being segmented they all have a cuticle (OVERHEAD) on the outside that is waterproof and strong. This forms an exoskeleton which provides all sorts of shields, legs, pincers, antennae, jaws, grinding surfaces, lenses. Having an exoskeleton means that arrangements for muscles are rather different (OVERHEAD) since the animal is, in effect, contained in a box. This means moulting - periodically shedding the cuticle so as to make way for a larger one. The effects of regular moults are profound : they allow repair of any damaged parts and modification of shape as well as increase in size.

 

Specialisation of appendages is easy to see if we compare an unspecialised crustacean and a lobster (OVERHEAD). Internally we see (OVERHEAD) the usual gut and nervous system made up of a ganglion around the oesophagus, segmental ganglia (with segmental nerves) in a ventral nerve cord. The main body cavity, however, is not segmented, or at least has no septa. This allows an open circulatory system with blood entering apertures in the pericardial sac around the heart, then being distributed to tissues by means of dorsal and ventral arteries. This blood eventually enters the large body cavity or haemocoel, and returns to the pericardium. Insects are like this too, but spiders have lost most of their external segmentation, though it is clearly seen in juvenile scorpions Otherwise they are clearly arthropods with modifications concerned with taking in liquid food and producing silk.

 

 

 

9. Starfish, sea urchins and sea cucumbers, the Phylum Echinodermata (6,000 species).(OVERHEAD)

Bauplan: bilaterally symmetrical larva, adult lacks head, pentamerous symmetry, tripartite coelom, internal mesodermal skeleton with calcium carbonate spicules, no excretory organs.

 

Just when you are beginning to see a clearish relationship developing, with bilaterally symmetrical segmented invertebrates of varying sophistication this comes along (OVERHEAD). Starfish have a central disc from which radiate a number of arms, usually five or a multiple of five. The mouth is in the centre of the underside, there is no head and any arm can act as the front end on the move. Locomotion is by taking in water via a series of sieve plates and transferring it via stone canals, ring canal and radial canal to tube feet with suckers (OVERHEAD). The gut is fairly rudimentary, the anus hardly used and there is no specialised excretory system, waste being absorbed by amoeboid cells. There is no brain, just a nerve ring around the mouth and a nerve cord in each leg connecting to a dorsal nerve net. Sea urchins are just starfish with the arms sewn up.

 

So why include this rather bizarre set of animals here, with the sophisticated invertebrates? As a matter of fact they show considerable resemblance to the chordates, which include you and me. The coelom in both chordates and echinoderms buds off from the gut. The echinoderm, incidentally has a brief bilaterally symmetrical stage before becoming secondarily asymmetrical.

 

 

10. You and I: Phylum Chordata (45,000 species)(OVERHEAD)

Bauplan: single dorsal nerve chord, notochord, gill slits, bilaterally symmetrical, triploblastic, well developed digestive tract, sense organs, sexual reproduction.

 

The simplest and most straightforward members of our own group, the chordates are small, inconspicuous animals of interest only to the Chinese who collect them by the ton for food. These are the lancelet, Amphioxus (OVERHEAD), meaning sharp at both ends.

 

Characteristic of all chordates is the notochord, a flexible skeletal rod that develops from the roof of the gut. This is a flexible tough endoskeleton on which the segmentally arranged muscles act. Both muscles and notochord extend beyond the anus as a tail. Another novelty is the single dorsal hollow nerve cord: most nerve cords we have seen so far are ventral and paired. A third feature traceable throughout most chordates is the presence of gill slits in the side of the pharynx. (OVERHEAD) The beasts filter feed: the tentacles keep out large particles, a ciliated groove in the floor of the pharynx (which is oddly the forerunner of our thyroid gland) secretes mucus which migrates up the wall to another ciliated groove in the roof, whence mucus and bits go backwards to the gut. Waste water goes out via the slits. The liver is formed, as in you or I, by an outpushing of the gut: it also has a portal system. There is no heart: blood is pumped round the closed circulatory system by contractions of the vessels, including a large ventral one which develops elsewhere, by folding, into a heart.

 

Not all chordates are as straightforward as this. The tunicates (OVERHEAD) are sedentary and therefore have enlarged the pharynx at the expense of other bits they also have a mainly radial symmetry: they do, however, have a heart and a free swimming larva (OVERHEAD) which is bilaterally symmetrical, and has a notochord, dorsal nerve cord and pharyngeal slits. Another unlikely chordate is the acorn worm, Balanoglosus (OVERHEAD) which is clearly chordate because of its pharyngeal slits: when sectioned this proves to have two nerve cords, one dorsal, sometimes hollow, one ventral. The other claim to fame of Balanoglossus is that its free swimming larva is remarkably similar to that of some echinoderms.

 

Much more familiar are the vertebrates proper. Of these the strangest are the hagfish and lampreys, jawless vertebrates showing a variety of features which place them as ancestral vertebrates. These survivors are specialised in many ways (one of the two main types is parasitic) but earlier agnatha (OVERHEAD) look like other fishes. Following on from these we just need to list the remaining types of vertebrates, the cartilaginous and bony fishes (SLIDE), the amphibia (SLIDE), the reptilia (SLIDE) the birds (SLIDE) and the mammals (SLIDE) of which man is a member.


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