The phylum family comprises different species having different habitats. Many of these species follow the ground-level living style, and some depend on their host to reach their adult stage. Most of the species in this family lead marine life, either in freshwater or salty seawater. This is because water life is a safe option for these species. These organisms form a jelly shape with eight external vertical ciliated comb plates. 

Protochordata

Protochordata is an informal animal classification system (i.e., not a taxonomic classification system) used to define invertebrate species that are closely related to vertebrates. The organisms belonging to Phylum protochordata are often called lower chordates. These groups of animals do not form a major part of Chordata. They do not have a true skull or cranium and, hence, are called Acraniata. They are classified into two subclasses: Tunicata or Urochordata and Cephalochordata. The protochordata is different from vertebrates due to the absence of a backbone. Recently, studies showed that protochordata may have descended from the same ancestral stock that produced vertebrates.

Characteristics of Phylum Protochordata 

• They live in marine water.
• They are triploblastic with three germ layers: ectoderm, mesoderm and endoderm.
• They are coelomate animals having a body cavity.
• Their body is bilaterally symmetrical.
• They have an organ system level of organisation.
• At some stage of their life or in some cases throughout their lifespan, these organisms form a notochord, which is a long and rod-like structure that provides support.
• The central nervous system of protochordata is hollow, solitary, and dorsal, in contrast to the central nervous system of non-Chordata animals.
• Most protochordata have a post-anal tail that ensures balance.
• The protochordata’s pharynx is a hollow tube that begins behind the nose and runs down the neck and is pierced with gill slits to allow water to circulate for respiration.
• These species have a closed circulatory system and have a ventral (frontal) heart.
• In protochordata, the gut (stomach) is on the ventral side of the nerve cord.

Some examples of protochordata are Amphioxus, Herdsman, and Sea salps.

Subdivision of Phylum Protochordata

The phylum protochordata is subclassified into two subphyla based on the type of notochord the organism possesses. They are: 

• Urochordata or Tunicata
• Cephalochordata

• Urochordata (Tunicata) 

Urochordata are filter-feeding marine animals that are mainly sessile (attached directly by the base without the help of a peduncle or stalk). The body, or Urochordata, is encased in a leathery sheath known as tunicates because it is made of a material known as tunicin.

Due to retrogressive metamorphosis, the notochord only appears in the larval tail of urochords and then vanishes in adults.

Basic Characteristics of Urochordates

• They are found mainly in marine environments.
• They are sessile.
• They are filter feeders.
• They do not have an excretory system.
• Their blood contains endocytic.
• They possess an open circulatory system.
• Their reproduction is asexual and carried out by budding.
• Gill slits or stigma, which is abundant in Urochordata creatures, allow them to breathe. Instead of opening to the outside, they open into the atrium.
• In adults, one dorsal ganglion replaces the nerve cord (also known as the neural tube in larvae).

The best examples of urochordates or tunicates are salpa, doliolum and herdsman.

• Cephalochordata

Branchiostoma, the little sand lance, belongs to the Cephalochordata subphylum. With fins, a tail, muscle blocks called myomeres, and pharyngeal gill openings for filter eating, this marine species resembles a fish. By examining their anatomy, we can notice their Chordate features. The larva resembles a miniature adult without gonads. This is the only group that keeps its Chordate features throughout its life.

 Basic Characteristics of Cephalochordates

• They are mostly found in marine environments.
• In Cephalochordata, excretion occurs via paired protonephridia (a network of dead-end tubules with no internal openings) and solenocytes (a type of flagellated cells).
• The tails of Cephalochordata stay throughout their lives. The Cephalochordata’s body wall is made up of myotomes (muscles fed by the spinal nerve), giving it a fish-like appearance. 
• Cephalochordata lacks a nerve cord and so lacks the ability to create a brain. 
• Their notochord stays with them for the rest of their lives, reaching up to the cephalic region (head).
• They have a lot of gill stilts (open in the atrium) that they keep even when they reach adulthood. 
• Cephalochordata is a free-swimming species that lives by burrowing.
• Their body walls possess myotomes (the muscles that a spinal nerve root serves).

The main example of Cephalochordates is Amphioxus or Lancelet.

Conclusion 

Protochordata is the group of animals that possess a notochord (a rod-like long structure) that may be present either during the embryo’s life or will be present throughout their lifespan. This notochord gives support to the body of the organism. Phylum protochordata is divided into two categories: subphylum Urochordata or Tunicata and subphylum Cephalochordata. This division is based on the type of notochord they possess. The main characteristics of organisms belonging to phylum protochordata are that they live in marine habitats, are mostly sessile and have their food using filter feeding. They are triploblastic, coelomate animals with bilaterally symmetrical bodies. The main examples of phylum protochordata members are Salpa, Doliolum and Amphioxus.