Problem 156 - Objective Biology for NEET

Question

In which subphyla, the notochord is present in tail of larva only? (a) Hemichordata (b) Urochordata (c) Cephalochordata (d) All of these

Step-by-Step Solution

Answer

The notochord is present in the tail of the larva only in subphylum Urochordata, so the answer is (b) Urochordata.

Step 1: Understanding and Identifying subphyla

Phylum Chordata can be divided into three subphyla which are Hemichordata, Urochordata, and Cephalochordata. Hemichordata are worm like marine organism and not considered to be a part of Chordata by many taxonomists, species in the subphylum Urochordata have a notochord at some stage of their life cycle, while Cephalochordata all possess a notochord throughout their life.

Step 2: Identifying the Notochord Presence

Notochord is one of the shared characteristics of all Chordates. However, in Urochordata, also known as Tunicates or Sea Squirts, the notochord is present only in the larval stage and is mostly confined to the tail. Thus, it can be concluded that the notochord occurs in the tail of larva only in case of subphylum Urochordata.

Key Concepts

Notochord

The notochord is a flexible, rod-like structure that plays a crucial role in the development of chordates. It provides support and defines the basic body plan of the organism. Throughout the evolution of chordates, the notochord has been a defining feature. It runs along the length of the body, serving as a primitive backbone. In more advanced vertebrates, the notochord is replaced by the vertebral column during development. However, during the embryonic stage, it is crucial for signaling and structure.

In some chordates, such as in the subphylum Urochordata, the notochord only appears temporarily. This provides insight into the amazing diversity within the chordate lineage, showcasing different adaptations and stages of development. Understanding the notochord's function enlightens us on the evolutionary significance of this structure across different species.

Subphylum Urochordata

Subphylum Urochordata, commonly referred to as tunicates or sea squirts, represents a fascinating group within the chordates. These marine organisms initially exhibit chordate characteristics only in their larval stage.

What makes Urochordata particularly interesting is their drastic metamorphosis. As larvae, they possess a notochord, a dorsal nerve cord, and a post-anal tail, all of which are typical chordate features. However, upon reaching maturity, many of these features are lost.

Adult tunicates do not have a notochord or dorsal nerve cord. Instead, they lead a sessile lifestyle, often attaching themselves to rocks or submerged surfaces. This unique adaptability highlights the evolutionary pathways available within the chordates, which can result in significant anatomical changes even within a single life cycle.

Phylum Chordata

Phylum Chordata encompasses a diverse group of organisms characterized by possessing a notochord at some stage in their life cycle. This phylum includes not only Urochordata but also other subphyla such as Cephalochordata and Vertebrata.

Among the defining features of chordates are the notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail. These critical structures serve various functions, ranging from support and movement to feeding and respiration.

The phylum boasts extraordinary diversity, with members adapted to an array of environments and ecological niches. Understanding Phylum Chordata offers insights into the complexity and adaptability of life forms, revealing the evolutionary origins of more complex vertebrates, including mammals, birds, and reptiles.

Larval Stage in Tunicates

In the life cycle of tunicates, the larval stage is crucial and fascinating. During this stage, tunicate larvae exhibit the fundamental characteristics of chordates, making them more closely resemble vertebrates than their adult forms do.

The larvae are free-swimming and possess a notochord, a dorsal nerve cord, and a post-anal tail. These features equip the larvae with the necessary structure and ability to swim and disperse before they settle down. This period of motility is essential for their survival and the spread of the species.

When the larvae transition to adulthood, they undergo metamorphosis. This process results in dramatically altered anatomy and physiology, with the sedentary adult form often losing many chordate traits. This metamorphosis marks a significant shift from an active, motile phase to a stationary one, illustrating the unique life strategy embraced by tunicates.