As it has been a while since we have been on the evolutionary journey, here is a link to the last post in the series. So far, we have got to wormey things with shells, and today we take a step into a world full of shells, that of the molluscs. I am not doing land based molluscs yet, as they do not turn up in our little journey for a while yet, and the sea has more than enough cool animals for now!
Molluscs are a HUGE phylum, with 90,000 species living, and 70,000 fossil species, so I am going to break this section down into segments. I will start with Polyplacophores (Chitons), then move onto Bivalves, then Gastropods (marine only), before finishing with Cephalopods. I will do two posts on each group, one outlining their habitat and morphology (body shape and structure) and one on particularly interesting features of each group. I could write a load of posts on each group, but I will be coming back to them when I do future series on sea-life, so this will just be an intro to whet your appetite, and maybe show you some cool stuff you did not know.
So…lets get started by meeting the Polyplacophores (Meaning “carrier of many plates”):
These beautiful creatures are found in shallow waters globally, generally up to 90m deep , and the bright colours of the ones I have shown above is to blend in with their surroundings (There are several species with more “boring” brown and green exteriors, but I thought I would show the pretty ones here!)
So, what do they look like on the inside?
By now in our journey through organisms, we have run into many of the terms in the diagram above, so there are fewer of them which are scary looking. (My functional biology professor would be impressed at me remembering these terms, I truly sucked at it in lectures!)
We have already run into “pedal” several times, meaning foot, “Nephridiopore” is the same as when we had nephridia, which we encountered when we first met bilateral animals, and relates to excretion. A nephrologist is a doctor who specialises in kidneys, our excretory organ. Gonad and Gonopore relate to sexual organs (the name is not specific to either male or female). The dorsal artery is the “top artery”, remember sharks have dorsal fins on their top side.
One which may seem new, but is not, is haemocoel (or hemocoel). Remember how we had acoelomate animals here? That meant that they were without body cavities, and coel relates to a body cavity. Hemo relates to blood, so this is a cavity through which blood travels. These animals do not have a full network of capillaries, veins and arteries like we do, and the blood is pumped out from the heart (at the back end of the animal, labelled as Pericardial cavity, ventricle and left atrium), and travels through arteries into spaces in the body where it is distributed to the tissues before going back to the heart via a vein. This is called open circulation, and is very common in smaller animals. As organisms become larger, this becomes less efficient for transporting oxygen to tissues, and so networks of capillaries develop to better transport the blood around the body.
The radula is a new term, and it is a specialised feeding organ that molluscs have, and we will be covering that in detail in the next post, as it is really awesome.
The mantle is the part which the shell is secreted from, and is actually like a layer of skin which hangs down over the sides of the animal, protecting the internal organs.
As you have noticed in the pictures above, these animals do not appear to have one continuous shell, but rather resemble woodlice (albeit brightly coloured ones). This is not because woodlice are in the same phylum, or even class as these, as woodlice are crustaceans (yes, the same group (subphylum) as crabs and lobsters). Rather, it is because the shell on these animals and on woodlice serves the same purpose, to allow maximum movement while giving protection.
I think most people have seen a woodlouse curl up when it is in danger (or when we try to pick it up to move it). These do the same, and they could not do that if they had a single, solid shell as other molluscs do. There are many ways in which a shell can protect an animal, and the each way of using a shell occurs in several distinct species. Some species of Armadillo have plates which allow them to roll up in this manner. As it has evolved in several different organisms, it is clearly an effective defense strategy!
I will leave you with a video of one of these animals moving around, and next post will cover the radula, and some other features which I find fascinating about these creatures.