Protochordates

Summary

This educational video explores the lesser-known but biologically significant group of proto chordates, which includes sea squirts (tunicates) and lancelets. Both are part of the phylum Chordata, sharing key characteristics like a notochord and dorsal nerve cord. The video delves into their anatomy, highlighting the lancelet's segmented muscles and the tunicate's filter-feeding system. It also discusses the new taxonomic classification that separates them into subphyla, reflecting recent molecular evidence.

Takeaways

• 🐠 Protochordates, including lancelets and tunicates, are crucial for understanding the early evolution of the phylum Chordata, to which humans belong.
• 📚 Both lancelets and tunicates are part of the invertebrate chordates and are studied in the invertebrates 3 laboratory exercise.
• 🔍 All chordates share four key characteristics at some point in their life cycle: a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a muscular post-anal tail.
• 🏷️ Protochordates are classified into two subphyla, reflecting a new taxonomic category between the phylum and class levels.
• 🧬 Molecular evidence has led to revisions in the classification system, distinguishing lancelets in the subphylum Cephalochordata and tunicates in the subphylum Urochordata.
• 🔬 Lancelets, or amphioxus, have a simple body structure with a dorsal fin, caudal fins, and a segmented body resembling early chordates and annelids.
• 👀 Lancelets lack eyes but have simple eye spots on the nerve cord for light detection, and they do not have paired appendages like pectoral or pelvic fins.
• 🍽️ Lancelets are filter feeders, using cilia and mucus to capture microscopic organisms and transport them to the stomach for digestion.
• 🌊 The pharynx of lancelets has gill slits that allow water to pass through, facilitating respiration and the filtration of food.
• 💊 The hepatic cecum in lancelets is believed to be homologous to the liver in vertebrates, playing a role in digestion and nutrient absorption.
• 🔄 Tunicates, as adults, are sessile and filter feed, losing certain chordate characteristics, but their larval form exhibits all the defining features of the phylum.

Q & A

What are the two groups of organisms referred to as proto chordates or invertebrate chordates?

The two groups of organisms referred to as proto chordates or invertebrate chordates are sea squirts, also known as tunicates, and lancelets.

Why are proto chordates important in biology?

Proto chordates are important in biology because they help us understand the early evolution of the phylum Chordata, to which vertebrates, including humans, belong.

What are the four derived characters shared by all chordates at some point in their life cycle?

The four derived characters shared by all chordates at some point in their life cycle are a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a muscular post-anal tail.

What is the difference between the subphylum Cephalochordata and Urochordata?

Cephalochordata is the subphylum that includes lancelets, while Urochordata includes tunicates or sea squirts. Both are part of the phylum Chordata but have different characteristics and life cycles.

Why did taxonomists revise the classification system for chordates?

Taxonomists revised the classification system for chordates due to molecular evidence that suggested all groups should be monophyletic, leading to a reevaluation of the relationships among chordate groups.

What is the common name for lancelets and why is it called that?

The common name for lancelets is 'lens' or 'lentils', due to their overall shape, which is similar to the shape of a lentil, and the no longer used genus name amphioxus, meaning pointed at both ends.

What are the structures known as myomeres in lancelets?

Myomeres, also known as segmental muscles, are visible through the thin epidermis of the lancelet. They are arranged similarly to that seen in fish and provide evidence that the earliest chordates had a segmented body.

How do lancelets feed and what is the role of the wheel organ in this process?

Lancelets are filter feeders that depend on cilia to draw water and food into their digestive system. The wheel organ, located at the front of the velum, along with the inner surface of the pharynx, is covered in cilia that help create a current to draw in microscopic organisms.

What is the function of the hepatic cecum in lancelets and how is it related to the liver in vertebrates?

The hepatic cecum in lancelets is a blind-ended pouch that secretes enzymes and takes part in the absorption of nutrients. It is considered homologous with the liver in vertebrates, indicating a shared evolutionary origin.

How do adult tunicates differ from their larval form in terms of the presence of chordate characters?

Adult tunicates, being sessile, lose certain structures, including three of the four derived characters of the phylum Chordata. However, their larval form reveals all four expected characters, indicating their chordate nature.

What is the role of the pharynx in the feeding process of adult tunicates?

In adult tunicates, the pharynx, which occupies most of the body, functions as a filter-feeding apparatus. Water is drawn in through the incurrent siphon, and as it passes through the pharynx, planktonic creatures adhere to a mucus coating, which is then transported to the digestive system.