So, last time was a quick introduction to Brachiopods, a bit about their fossil record, and possible suggestions for why there was a decline in the number of species (diversity).
Today is going to be about modern Brachiopods. First of all, we should have a look at what they look like inside, and their body shape (morphology)
Now, there are a ton of terms on there, with varying degrees of familiarity (I think everyone knows what a gut is, and from previous posts, we have covered body cavities (coelom), and nehphridia (excretion systems, similar to kidneys) in the posts on Platyhelminthes) but only a few of these terms are relevant to this post, so it is not as overwhelming as it can look!
The pink sticky-uppy thing at the top of the shell is labelled as a pedicle. When you see a label in anatomy with ped at the front of it, this usually refers to a foot (We are bipedal, this means we have two feet). This is attached to the ventral (lower) valve. It confused me when I first saw one of these anatomical diagrams, as it appears that they have drawn it upside down. When you look closer however, the pedicle is attached to, and emerges from, the lower valve.
This pedicel is used to attach to the sea floor or rocks etc, so is usually on the bottom of the organism. In some species, this is attached at the bottom of a vertical burrow, as in this diagram
Here is an image showing one in its burrow
The next thing that you can see on the diagram is that the body cavity only takes up a small part of the inside of the shell. Around two thirds of the shell interior is taken up by something labelled as a lophophore. This is quite a difficult thing to describe.. The picture below shows a brachiopod with its shell open, and the lophophore is the fringey thing you can see taking up most of the space inside.
For comparison, here is a brachiopod fossil, with the lophophore structure nicely fossilized
Here is a another brachiopod fossil, with the lophophore being the white coil in the bottom left
Ok, so what is this weird thing taking up so much shell space?
The best way to think about a lophophore is as an all in one eating and breathing tentacle system. In Brachiopods, this is horseshoe shaped. The fringey looking things are cilia, which help in drawing currents of water over the tentacles, the tentacles catch the food particles and transport them to the digestive system. This is done in grooves in the tentacles, and there are grooves for transporting food in, and transporting unwanted food back out.
This is sometimes difficult to visualize, so I hope these pictures below help:
This is a diagram which I think more clearly shows the lophophore, and its connection to the digestive system
This next image is a bit more techy, but it shows a dye stream in water entering, then being expelled from a brachiopod.
The food particles are carried into the opening of the stomach, where they are digested, and any waste is excreted from the end of the stomach, where it is removed from the body cavity into the mantle cavity by the nephridia, and then the cilia carry this waste out of the shell opening.
Because Brachiopods have a shell, they are able to control the flow of water over the cilia, there are other organisms which have lophophores, but no shells, and we will be covering these in the next post.
Apologies for the brevity of this post, normal rambling will resume after my exam tomorrow!