Welcome, class! Today, we are discussing the Phylum Hemichordata. Can anyone tell me what they know about this group?

Yes! Hemichordates have characteristics similar to chordates, but they are grouped separately due to unique features like the stomochord, which resembles the notochord. It's important to remember 'Hemi' means half, indicating they have some similarities but are not fully chordates.

Great question! They are typically worm-like with three main body sections: the proboscis, collar, and trunk. Picture a worm divided into these three parts when visualizing them.

They have gills for respiration. Just remember that in many aquatic animals, gills are the primary respiratory structures. So, Hemichordates breathe through gills!

They practice external fertilization. Imagine a water environment where fertilization happens outside the body! Their development is indirect, which means they go through distinct larval stages.

To summarize, Hemichordata is a fascinating phylum with unique features, including the stomochord, and they exhibit an open circulatory system. Let's keep our eyes out for specific examples, which we will talk about next.

Now, let's explore more about the body structure of hemichordates. What are the three main parts that comprise their body?

Exactly! The proboscis is at the front for feeding, followed by the collar, which supports the stomochord and then the trunk, where the gut and other organs are located. Remember the mnemonic 'PCT' for Proboscis, Collar, Trunk!

Excellent question! Their cylindrical shape allows them to burrow into sediment or sand, providing protection from predators. This anatomy also aids in movement in aquatic environments.

Absolutely! Each part is perfectly suited for their lifestyle. As a recap, hemichordates have a three-part body structure that aids in feeding, movement, and protection.

Let's shift gears and discuss their circulatory system. Who can tell me what type of circulatory system hemichordates possess?

That's correct! They have an open circulatory system, meaning their blood is not confined to vessels but instead bathes organs directly. It’s a more primitive system found in several marine organisms.

Good observation! They breathe through gills, which efficiently extract oxygen from the water. This relationship between the system and their habitat is vital for their survival.

Certainly! Unlike hemichordates, chordates generally have a closed circulatory system. Remember this difference, as it highlights the evolutionary trends within the animal kingdom.

To wrap up, hemichordates use an open circulatory system for nutrient and waste transport and gills for respiration, fitting their aquatic environment well.

Today, we will look at the reproductive characteristics of hemichordates. How do they reproduce?

Exactly! This means that the male and female gametes meet outside of the body in the aquatic environment, which increases the chances of successful fertilization.

Great question! Their development is indirect, involving larval stages distinct from adults. This trait allows them flexibility in early life stages as they seek suitable habitats.

Yes! In many species, the larvae might have different structures and functions compared to the adult, which is vital for survival in different environments. To recap, hemichordates undergo external fertilization and have an indirect developmental cycle.