Join me on another night in the lab, taking a close and personal look at some of life's most slimy and sickening animals. Hey everyone!! I hope you enjoyed last week's segment where we took a deeper look at the anatomy and distinction between Porifera (Sponges) and Cnidaria (Jellies). There is a huge bulk of information here, so I'll make effort to keep it short and maybe come back to the classes of particular phyla once we've covered a larger portion of animals. I'm writing this just before going to write an exam for this lab actually, so this will serve as a two birds one stone scenario; studying and working at the same time!!
Today, we continue with worms galore! Continuing along with the invertebrates, we will be focusing on Platyhelminthes, Bryozoa, Nemertes, and Annelida, these being; Flatworms, Ectoprota, Ringworms, and Roundworms respectively. All of these phyla are deemed bilateria, as they are the first to develop this bilateral symmetry, which is unobserved in the previously covered sponges and radiates. Next week we will be looking at the Mollusks! So we'll be starting with long slimy things before big slimy things.
Platyhelminthes (Flatworms)
The first and most basic form of worm, Platyhelminthes are distinguished from other worms by their body shape. Being a "flat"-worm, as you can probably guess, means they have a flat body form. Usually, no more than 5cm in length, these bilateral and unsegmented, soft body acoelomates, have none of the specialized structures found in higher-order organisms. This means they lack a circulatory, respiratory and have a very simplified nervous system. They have adapted their flat body to act as a direct filter for their nutrients which are diffused directly through the organism from the ventral to dorsal surface. They are divided into four notable classes which are Turbelia, Planaria, Monogea and Trematodes.
Bryozoa (or Ectoprocta)
These interesting Protostome filter feed using their gills and are able to sieve out food particles through a retractable lophopore. Their exoskeleton is secreted from their body and is made from zoecium, stiffened with calcium. This zoecium exoskeleton also plays a part in the formation of the colonial form of Bryozoan, which is made of multiple 'individual' units called zooids connected through a pore plate. Usually hermaphroditic brooders, the atypical, asexual heterozooids are modified with bird-beak like heterozooids which feed instead of reproduce.
Nermertea (Ribbonworms)
Nemertes are a parasitic creed of worms and are often referred to as Ribbonworms. Freeliving and triploblastic (having three germ layers) and are another animal with bilateral symmetry, which will obviously become a common trait among many of the animals we will cover in the future. Moving on up in the animal world, they are the first organism to show a blood-vascular system and retain the simplest version of any animal. It is a closed loop circulatory system and is complemented by a complete digestive system.
This particular phylum is known for hosting the most common parasitic worm for people, the "large roundworm". These gross suckers will usually begin by infecting a mollusk of some sort from which its eggs can develop in this intermediate host. It then is able to disperse itself into the surrounding environment until it comes in contact with humans; the definitive host. From there they eat away at the host, as a parasite does, until it lays it's eggs while being transported through the host's feces back out to infect a new mollusk (don't worry we will get to them soon!). This cycle allows them to reproduce and infect a wide range of hosts.
Annelida (Roundworms)
And finally, we come to roundworms. Probably the most common of the worms for most people, the best example being the earthworm, these worms are distinguished from the others by their segmented body. Similarly, to the Platyhelminthes, Annelida there are three notable class distinctions within the phylum. These are the Oligochaeta, Polychaeta and Hirudinea which are commonly known as earth worms, marine worms and leeches.
Their more complex body plan allows more control over their motor functions which is aided by the circular annuli organs which allow it to stretch and longitudinal muscles allowing for contraction; these three together creating complex motion. They also have a hydrostatic skeleton, another unique feature which allows them to maintain body shape. Upon dissection, it can be observed that this earthworm, in particular, has bilateral symmetry of the organs all along the body, with a digestive track that covers the entire length of the worm.
IMAGE SOURCES:
REFERENCES:
[1] Flatworms
[2] Nemertes
[3] Protostome
[4] Annelida
[5] Worms
Remember guys, what goes around comes around! So be nice to these little fellas!
