Hello, my name is Danny Eibye-Jacobsen and I work at the Zoological Museum of the Natural History Museum of Denmark. The purpose of this video is to provide an overview of Metazoa, the multicellular animals, including the major features that characterize or define its major subdivisions. This should set the scene for the following videos on the Cambrian Explosion, during which many important animal groups appear for the first time in the fossil record. Biologists divide metazoans into about 35 so-called phyla, such as the arthropods, the molluscs, roundworms, echinoderms and chordates. Some of these phyla are grouped together based on characters that I will explain as we go along. In doing so, I will continuously refer to this highly simplified cladogram showing the relationships of the major groups. There are certain features that are common to all or almost all animals. First of all, they are heterotrophic, which means that they have to acquire organic matter from other sources in order to survive, generally by eating other organisms. Secondly, they are usually motile at some stage during their life cycle. Finally, in almost all metazoans early development goes through what is called a blastula stage. Let us focus on this blastula for a moment. The fertilized egg divides several times and in the simplest case, where there aren't a lot of yolk particles in the egg, the embryo reaches a stage in which a hollow ball of cells is formed which is called a blastula. The liquid inside the blastula is called the blastocoel. In all multicellular animals except sponges, further development involves the formation of a gastrula by the process of gastrulation. Gastrulation can take many forms, all leading to the same result, but the simplest method is the one shown here, called invagination. A group of cells invaginate from the surface of the blastula, forming a short, blind-ending tube into the animal. This is now a simple gastrula and the new opening is called a blastopore, leading into this invagination, which is called an archenteron. Through this simple process, the animal now has two so-called germ layers, an outer ectoderm and an inner endoderm, the cells that surround the archenteron. By forming an archenteron, the animal has created a more or less closed space that it can control and this part of the body typically develops into the digestive tract. The ectoderm on the outside covers and protects the body and it will later contribute the cells that will form the nervous system. I will return to the further development of the gastrula later, but it is time to take a closer look at the major subdivisions of Metazoa. There is general agreement that sponges, the Porifera, have the simplest body organization among multicellular animals. They exhibit the so-called parazoan body plan in which there are two layers of cells, separated by a layer of gelatinous material, the mesenchyme, that is not cellular, although it usually contains isolated cells moving through it. Sponges contain several different kinds of cells, but they are not generally organized into specialized tissues or organs. All other multicellular animals belong to a group called Eumetazoa, the true animals. The arisal of this group was one of the major steps in animal evolution. Eumetazoans all go through a gastrula stage and thus at a minimum they have an ectoderm and an endoderm. Another significant feature is that they develop specialized tissues, one of the most important of them being a true nervous system. Another characteristic of eumetazoans is that they have dedicated muscles cells, by which I mean cells that have no other function than to carry out muscular contractions. Eumetazoans are divided into two major subgroups: Radiata and Bilateria. As the name suggests, Radiata contains animals that are radially symmetrical, which means that they have a central axis and similar parts of the body are arranged symmetrically around this axis. This arrangement works very well for animals that are sessile or planktonic, because they live in environments that are bascially the same in all directions. Two phyla belong to the Radiata, the cnidarians, such as jellyfish and sea anenomes, and the ctenophores or comb jellies. The basic organization of the body is that of a gastrula in that they have an outer ectoderm and an inner endoderm with one opening to the exterior, the mouth. Since there are only these two germ layers, we call this a diploblastic organization. Similar to what we saw in the sponges, these two cell layers are separated by a gelatinous layer that in this case is called a mesoglea. The mesoglea is secreted by the two cell layers and contains isolated, wandering cells. The mesoglea can be very thin, but in jellyfish it constitutes almost the entire mass of the animal. We have now looked at sponges, cnidarians and ctenophores. All other animals belong to the Bilateria, containing almost everything that you would typically think of when talking about animals. Bilaterians are bilaterally symmetrical, as we see in this fish. This means that they have a defined anterior end and a posterior end. They also have a dorsal side and a ventral side. So: front, back, up and down are defined, which automatically gives the animal a right side and a left side. Such animals, especially if they are mobile, can have much more complicated interactions with the surrounding environment than radially symmetrical animals can. What makes bilaterian animals so complex can to a great degree be explained by the concept of compartmentalization. Bilaterians usually have lots of more or less closed compartments and this has allowed them to develop increasingly specialized tissues and organs. For example, true circulatory systems, with blood vessels, and true excretory systems are only found in bilaterians. There are many cases where such organs are not present, particularly in small animals, but this is generally thought to be the result of secondary reduction. The fundamental basis for this compartmentalization – and thus this new level of complexity – is that bilaterian animals have a triploblastic organization. This means that they have three germ layers. In addition to the ectoderm and endoderm that we know from radially symmetrical animals, they have a layer of cells called the mesoderm between the ectoderm and endoderm. In many bilaterians – but not in all of them – the mesoderm surrounds a fluid-filled cavity called a coelom, or sometimes several of them. The processes of mesoderm formation and coelom formation are central to understanding the unparallelled success of the bilaterians. There are a number of ways that these processes can take place, but there are two major variations which are the basis for the main subdivision of Bilateria into Protostomia and Deuterostomia. Let's look first at the deuterostomes. This group includes echinoderms like sea stars and sea urchins and it also contains the chordates. The largest group of chordates are the vertebrates, including fish, snakes, birds and mammals like us. To understand mesoderm formation and coelom formation in deuterostomes we have to return to embryology, specifically in echinoderms, where they can be observed in their simplest form. When the fertilized egg divides in the process called cleavage, the first two divisions lead to the formation of four identical cells. This is common to all bilaterians, but the next cleavage, which is always perpendicular to the first two cleavages, leads to two levels of four cells. In the deuterostomes the cells of the upper level are directly above those of the lower level and this tendency continues during further cleavages. This pattern is called radial cleavage. Animals with radial cleavage have what is called regulative development. If the individual cells of, for example, the four-cell stage are separated from one another, each one will start over and make a perfect clone of the animal, for example a sea urchin. Regulative development means that the fate of each cell is determined by its interactions with its neighbouring cells. If the cells are removed from one another, there are no such interatctions and each cell behaves like a fertilized egg. By the way, this only works up to about sixteen cells. After that, the cells have begun to specialize and can't start over again. Further cell division leads to the formation of a gastrula. However, in deuterostomes and in the other bilaterians, the protostomes, something new happens at this stage. The archenteron goes from being a blind-ending invagination to being a complete tube by breaking through the ectoderm at the opposite side of the gastrula from the position of the original blastopore. By this simple innovation, a complete digestive tract is formed, with a mouth and an anus. The food that the animal eats only has to move in one direction, making digestion much more efficient, a major step forward for the Bilateria. Getting back to the deuterostomes specifically, what happens next is that the original blastopore becomes the anus of the adult animal or closes. The second opening becomes the mouth and this is the basis for the name Deuterostomia, which means second mouth. If necessary, the archenteron will later break through the ectoderm again in a new position, forming the anus of the adult. We now return to the central theme of Bilateria, the formation of a mesoderm and a coelom. In deuterostomes this takes place simultaneously by a process called enterocoely, where invaginations from the archenteron form pockets that finally separate completely from the archenteron and lie between the ectoderm and endoderm. The walls of these pockets transform into the mesoderm, which will later form almost all the internal organs of the animal. The liquid inside these pockets becomes the coelom, which has various functions in transport, excretion and reproduction and which may provide the animal with rigidity and structural stability. Let us now turn to the other main subgroup of Bilateria, the protostomes, and their embryology. Protostomia is further divided into two major subgroups, which I will return to, but in one of these, the Ecdysozoa, the eggs are so rich in yolk that their embryology is strongly modified and often highly complicated. In the other group, the Lophotrochozoa, the amount of yolk in the egg is usually smaller and it is much easier to follow their embryology, which is what we will do. We will compare it with the processes that we have just talked about in the deuterostomes. First of all, cell cleavage beyond the first four cells takes place differently. When eight cells are formed by transverse division, the upper four cells rotate 45 degrees to rest in the spaces between the cells of the lower level. This continues during subsequent cell divisions, but always in the opposite direction of the last time: 45 degrees to the right, 45 degrees to the left, right, left, and so on. This is called spiral cleavage, unlike the radial cleavage we saw in deuterostomes. If we do the same experiment as before, where we separate the first four cells of the embryo from one another, each cell will develop into a deformed, non-functional larva that cannot survive. The reason for this is that even in the unfertilized egg of a protostome, a multitude of chemicals are distributed unevenly within its cytoplasm. This means that when it is fertilized and cell cleavage begins, even the first cells are not identical. The fate of each cell is determined by the unique concentrations of substances that it contains and it is destined to form a specific part of the body that would be absent if that cell were to die or be removed. In deuterostomes we had regulative development – here we have what is called mosaic development or determinate development. After a number of cell cleavages a gastrula is once again formed and, as in the deuterostomes, the archenteron breaks through the ectoderm to form the beginning of a complete digestive tract. However, unlike in the deuterostomes, the original blastopore will become the mouth of the adult and the new opening will become the anus. This explains the name Protostomia – first mouth. Unlike what we saw in deuterostomes, the formation of mesoderm and coelom is not simultaneous in protostomes. After gastrulation is complete, the embryo starts to lengthen and two specialized cells at the posterior end, called teloblasts, one on the right side and one on the left side, begin to grow and divide at a furious pace, forming bands of cells that are pushed forwards into the old blastocoel. This new tissue is the mesoderm and the process is called teloblastic mesoderm formation. Coelom formation was by enterocoely in the deuterostomes. In protostomes fluid-filled spaces begin to open up inside these aggregations of mesoderm cells as they expand and these spaces will develop into the coelom. This process is called schizocoely. In summary, we have radial cleavage and regulative development in deuterostomes, but spiral cleavage and mosaic or determinate development in protostomes. The mouth is a secondary opening in deuterostomes, whereas the original blastopore becomes the mouth in protostomes. Finally, mesoderm and coelom formation is simultaneous in deuterostomes by the process of enterocoely, but in protostomes a teloblastic mesoderm formation is later followed by coelom formation through schizocoely. Let's have a closer look at the Protostomia. There are two major subdivisions, the Ecdysozoa and the Lophotrochozoa. Ecdysozoans include animals like roundworms and the largest of all phyla, the arthropods. They are characterized by having an outer layer of cuticle that covers and protects the ectodermal epidermis and by periodically changing this cuticle through the process of moulting. Lophotrochozoa includes numerous phyla, for example flatworms, molluscs like snails and bivalves, and annelids like earthworms. It is difficult to find characters that clearly define the entire group and support for it comes mainly from molecular studies. However, many phyla in this group have a specialized kind of larva called a trochophore. It is also among lophotrochozoans that we see spiral cleavage in it purest form, which explains the alternative name that we have for this group, the Spiralia. The last thing I would like to do in this video is to show you this cladogram. It is somewhat different from the cladogram that we have been looking at up to now and it is just one of a multitude of competing hypotheses of early animal evolution. The only thing I want you to notice here is the great concentration of large black dots, corresponding in time to the Ediacaran and especially the Early Cambrian periods on the time line at the top of the figure. The dots symbolize the first known fossils of various animal phyla like sponges, arthropods, molluscs, annelids and chordates. This is not the same as the time at which these groups first arose and we use other tools to figure that out, including molecular studies, which generally push these dates much further back in time. However, a great number of animal groups appeared in the fossil record for the first time in the Cambrian period and the extraordinary circumstances that led to this are the subject of the following videos about the Cambrian Explosion.
