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Platyhelminthes are commonly known as flatworms. Some of the examples of Platyhelminthes include tapeworms and flukes. These flatworms usually exist as parasites which feeds on its host or it can exist as non-parasitic animals such as the planarians (Schmidtea mediterranea). On the other side, Annelida are annelids which include mostly earthworms, marine worms and leeches. These annelids are mainly detritivores which feed on sediments that contain microorganism. Moreover, there are also some other annelids that are hunters as they catch their prey with jaws. Further examination on the morphological characteristics, type of body cavity, internal systems and reproductive mechanism allows us to make more comparisons between these two fascinating phyla.
At the first glance, both flatworms and annelids share similar characteristics as bilaterian animals. Animals that are bilaterally symmetrical mean that their anterior and posterior ends are different. In addition to that, their dorsal and ventral surfaces are also different. (Refer to Fig 1 and 2) In comparison to Cnidarians and Cetenophores, Platyhelminthes and Annelids have a higher morphological and physiological organisation as they have developed organ systems. Both Platyhelminthes and annelids possess true tissues that are organised into three germ layers, namely the endoderm, ectoderm and mesoderm (triploblastic). The mesoderm layer gives rise to muscles, various organ systems and parenchyma tissues. (Alters, 2000) Additionally, both of them have displayed some degree of cephalisation by developing head and tail-like structures. Furthermore, these animals are soft bodied invertebrates and do not have any limbs. Hence, it is their morphological nature and structural characteristics that give rise to the similarities between Platyhelminthes and Annelida phylum group.
The characteristic mentioned above describe the structural similarities between the Platyhelminthes and the Annelida phylum group. However, despite the structural similarities, there are differences observed between these two phylum groups. In terms of body shape, Platyhelminthes are dorsoventrally flattened and have unsegmented body parts. Annelids, on the other hand, are cylindrical and display metamerism. (Refer to Fig 1 and 2) Although both Platyhelminthes and Annelida are triploblastic, Platyhelminthes are acoelomates, meaning they do not have true body cavity and their organs are in contact with epithelium. On the contrary, Annelida are eucoelomates, they formed true coelom with complete lining layer called peritoneum. (RussellHYPERLINK "#_ENREF_3" et al.HYPERLINK "#_ENREF_3", 2011)
Most of the flatworms and annelids share common similarities in their respiratory system. Both flatworms and annelids participate in gaseous exchange through gas diffusion in their skin layer. Generally, they respire at their integument as they do not possess any special organs for gaseous exchange. Comparing their circulatory systems, the annelids have a closed circulatory system which contains blood vessels near their skin surfaces. These blood vessels contain
blood which carries nutrient and oxygen throughout the body. Blood is pump by the contractile lateral vessel which acts like a heart within the gut. The flatworms, in contrast, do not have any blood vessels in their body and there is no specialised circulatory system within the internal body.
Most of the flatworms possess an incomplete digestive system which consists of the mouth, pharynx and an intestine with one opening. Similarly, the annelids also possess a mouth and pharynx. However, it has a rather complete digestive system in which their intestine has two openings. The two openings in the intestine allow for food movement through the mouth and allows for waste excretion through the anus. When comparing excretory system in flatworms with that of annelids, various distinctions are discovered. The flatworms excrete their waste into the gastrovascular cavity and eliminate their waste through their mouth. On the contrary, the annelids have a pair of excretory glands known as metanephridia in each segment that excretes soluble wastes products. Add on to that, solid waste products can also be excreted through the worm anus. (RussellHYPERLINK "#_ENREF_3" et al.HYPERLINK "#_ENREF_3", 2011) Hence, both these phyla have different ways of waste excretion.
The flatworms have a simple nervous system which consists of two simple brains called ganglia. The ganglia are a bundle of nerves which connects to the longitudinal nerve cords and run throughout on the either side of the body. In addition, they have eye spots which allow them to detect for light signatures. (Alters, 2000) On the other side, the nervous system of annelids is rather complex in nature. The annelids consist of a dorsal brain with a pair of nerves on either side. They also consist of a pair of ventral nerve cords connected to ganglia in each segment. Furthermore, they also have transverse nerves attaching to the ganglia in each segment giving the complexity of the nervous system in annelids.
Both flatworms and annelids have also adopted similar reproductive strategies to ensure their survival across time. In their reproductive mechanisms, both flatworms and annelids are able to reproduce asexually or sexually. In asexual reproduction, both of them are able to reproduce through clonal fragmentation. When a flatworm or an annelid is split into fragments, each of these fragments will quickly regenerate and develop into a new flatworm. In sexual reproduction, both flatworms and annelids have hermaphroditic sexual organs which allow them to cross fertilise via copulation. After cross-fertilisation, the worm secretes a cocoon where the fertilised eggs are deposited. Such sexual and asexual reproductive strategies allow these worms to propagate and prevent them from extinction across the time.
The great diversity of nature is a result of a long history of evolution. Classification of animals according to their morphological characteristic provides a greater detail of explanation to the animal behaviour. The similarities in the morphological structure and reproduction systems between the Platyhelminthes and Annelida suggest their common ancestry. However, judging only from their morphology and reproductive system alone is insufficient to classify these animals. Hence, we need to look into other key characteristics that allow us to differentiate between Platyhelminthes and Annelida. One of the most important key characteristics that allow us to classify animals is genetics. Modern genetic techniques allow us to compare genetic sequences to reveal their common ancestry. Therefore, molecular data obtained from DNA will provide an accurate and useful tool for the identification of the animal phyla.
Alters, S., 2000. Biology: Understanding life. Jones and Barlette Publishers.
Moore, J., 2006. An introduction to the invertebrates. Cambridge University Press.
Russell, P.J., P.E. Hertz and B. McMillan, 2011. Biology: The dynamic science. Cengage Learning.
Table 1: shows the similarities and differences between Platyhelminthes and Annelida
Soft-bodied invertebrates, does not have limbs
Have a head and a tail
Acoelomates(without body cavity)
Diffusion through skin
Absence of blood vessels
No specialised circulatory system
Incomplete (gut has one opening)
Excrete waste by mouth
simple nerve ganglion, have eye spots that detect light
Both have asexual and sexual reproduction
Fig 1 shows the morphological structure of the annelids.
Fig 2 shows the morphological structure of the flatworm.