Difference Between Urochordata and Cephalochordata

Main Difference – Urochordata vs Cephalochordata

Chordata refers to an animal phylum characterized by the presence of pharyngeal gill slits, a notochord, a dorsal, hollow, nerve cord, and a post-anal tail. The three subphyla of the phylum Chordata are Urochordata, Cephalochordata, and Vertebrata. Therefore, the closest relatives of vertebrates are urochordates and cephalochordates. Both urochordates and cephalochordates are called protochordates. Urochordates are the more closely related group to vertebrates while cephalochordates are more closely related to echinoderms.  The main difference between Urochordata and Cephalochordata is that Urochordata consists of a notochord extended in the head region whereas Cephalochordata contains the notochord in the posterior region of the body.

Key Areas Covered

1. What is Urochordata
     – Definition, Characteristics, Structure
2. What is Cephalochordata
     – Definition, Characteristics, Structure
3. What are the Similarities Between Urochordata and Cephalochordata
     – Outline of Common Features
4. What is the Difference Between Urochordata and Cephalochordata
     – Comparison of Key Differences

Key Terms: Cephalochordata, Chordata, Cuticular tunic, Notochord, Protochordate, Vertebrata, Urochordata

What is Urochordata

Urochordata refers to a group of chordates comprising of tunicates. Tunicates are also called sea squirts. Around 200 living species of tunicates are found in the sea. The majority of tunicates are sessile. The most significant feature of urochordates is the presence of a notochord in the larval stage. The body of the adult urochordates is unsegmented and lacks a tail. The body is covered by a test or cuticular tunic. The test is formed by tunicine that is rallied to cellulose. Tunicates are shown in figure 1.

Figure 1: Didemnum molle

The body wall of urochordates is composed of an epidermis, dermis, and atrial epithelium. The epithelium of urochordates is made up of a single layer. The dermis is made up of muscles and connective tissues. The larval form of tunicates is tadpole-like and consists of pharyngeal slits, a hollow, dorsal notochord, and a muscular post-anal tail. The larval form is free-swimming. The pharynx and the internal gills of the adult are enlarged. Urochordates exhibit a ciliary mode of feeding. They have an open circulatory system. A single dorsal ganglion is present in the nervous system. Nephrocytes are responsible for the excretion of urochordates. The asexual reproduction of urochordates occurs by budding. Urochordates are bisexual and the external fertilization is the mode of sexual reproduction.

What is Cephalochordata

Cephalochodata refers to a group of chordates comprising of lancelets. The main feature of cephalochordates is the presence of a notochord that extends through the entire length of the body. But, they do not have a defined brain. The body of cephalochordates is fish-like and is adapted for swimming and burrowing. Adult cephalochordates have a head and a tail but, do not have appendages. Instead, dorsal, ventral, and caudal fins are present in the body. A cephalochordate is shown in figure 2.

Figure 2: Branchiostoma lanceolatum

The body wall of the cephalochordates is made up of a single-layered epidermis and a dermis made up of connective tissue, striated muscle, and parietal peritoneum. But, cephalochordates lack an exoskeleton as urochordates. Cephalochordates consist of a closed circulatory system with a hepatic portal system. They consist of two pairs of cerebral and several pairs of spinal nerves. The excretory system of cephalochordates consists of protonephridia and solenocytes. Cephalochordates do not undergo asexual reproduction. External fertilization occurs in cephalochordates as well.

Similarities Between Urochordata and Cephalochordata

• Urochordata and Cephalochordata are two subphyla of the phylum Chordata.
• Urochordata and Cephalochordata are the two most relative groups of vertebrates.
• Both Urochordata and Cephalochordata consist of pharyngeal gill slits, a notochord, a dorsal, hollow, nerve cord, and a post-anal tail.
• Both Urochordata and Cephalochordata lack well-developed sensory organs.

Difference Between Urochordata and Cephalochordata

Definition

Urochordata: Urochordata refers to a group of chordates comprising of tunicates.

Cephalochordata: Cephalochodata refers to a group of chordates comprising of lancelets.

Relatives

Urochordata: Urochordates are more related to vertebrates.

Cephalochordata: Cephalochordates are more related to echinoderms.

Position of the Notochord

Urochordata: The notochord of the urochordates can be identified only in the posterior part of the body.

Cephalochordata: The notochord of the cephalochordates is extended in the head region.

Occurrence of the Notochord

Urochordata: Urochordates consist of a notochord only in the larval stage.

Cephalochordata: Cephalochordates consist of a notochord throughout their life cycle.

Post-Anal Tail

Urochordata: Urochordates consist of a post-anal tail only in the larval stage.

Cephalochordata: Cephalochordates consist of a post-anal tail throughout their life cycle.

Coelom

Urochordata: Urochordates do not have a coelom.

Cephalochordata: Cephalochordates have a coelom.

Body Structure

Urochordata: Urochordates have an unsegmented body.

Cephalochordata: Cephalochordates do not have a segmented body.

Circulatory System

Urochordata: Urochordates have an open circulatory system with a heart.

Cephalochordata: Cephalochordates have a closed circulatory system. They do not have a heart.

Excretion

Urochordata: The excretion of urochordates occurs through nephrocytes.

Cephalochordata: The excretion of cephalochordates occurs through protonephridia and solenocytes.   

Free-swimming Animals

Urochordata: Urochordates are free-swimming during the larval stage while the adults are sessile.

Cephalochordata: Cephalochordates are fish-like, free-swimming animals.

Habitat

Urochordata: Urochordates are exclusively marine.

Cephalochordata: Cephalochordates are found buried in the sand of the intertidal zone of the coast.

Body Covering

Urochordata: Urochordates have a cuticular tunic or test.

Cephalochordata: Cephalochordates do not have a hard body covering.

Asexual Reproduction

Urochordata: Urochordates undergo asexual reproduction by budding.

Cephalochordata: Cephalochordates do not undergo asexual reproduction.

Sex Separation

Urochordata: Urochordates are bisexual or hermaphrodites.

Cephalochordata: Cephalochordates are unisexual animals.

Examples

Urochordata: Urochordates are tunicates.

Cephalochordata: Cephalochordates are Branchiostoma and Amphioxus.

Conclusion

Urochordata and Cephalochordata are two subphyla of the phylum Chordata. Thus, both urochordates and cephalochordates consist of pharyngeal gill slits, a notochord, a dorsal, hollow, nerve cord, and a post-anal tail. Urochordata consists of a notochord in the posterior part of the body while Cephalochordata consists of a notochord extended to the head region. The main difference between Urochordata and Cephalochordata is the presence of the notochord in the head region.

Reference:

1. “GENERAL CHARACTERS OF UROCHORDATA.” BioScience.pk – Read, Available here.
2. “GENERAL CHARACTERS OF CEPHALOCHORDATA.” BioScience.pk, Available here.

Image Courtesy:

1. “Sea Squirts Didemnum molle” By Silke Baron – originally posted to Flickr as Sea Squirts (CC BY 2.0) via Commons Wikimedia
2. “Branchiostoma lanceolatum” By © Hans Hillewaert (CC BY-SA 4.0) via Commons Wikimedia