Biology 2nd Paper (Zoology) · Classes 11-12 · Chapter 1

Phylum Chordata and its Subphyla.

A hilsa, a hen and a human all belong to one phylum. So does a baglike animal stuck to a rock. What do they share? The answer sits in the embryo: four marks that every one of them carries at some point in life.

1. Who the chordates are

Chordata comes from Latin chorda (cord) and Greek ata (to bear). Bateson named the phylum in 1885. So far 68,626 living species have been identified. Every animal from the tiny anchovy to the human is in it. The two phyla most closely related to it are Echinodermata and Hemichordata.

The definition fits in a sentence: an animal that has a rod-like, elastic notochord along the dorsal midline at some stage of life is a chordate. Its nerve cord runs along the back and is hollow, gill slits lie on both sides of the pharynx, and the heart sits ventrally, on the belly side.

2. Four marks in the embryo

The notochord (noton = back + chorda = cord) lies along the middle of the back in the embryo or throughout life. In advanced animals the vertebral column replaces it in the adult, and we call such an animal a vertebrate. The nerve cord lies right above the notochord, hollow and tubular. In vertebrates it becomes the brain in front and the spinal cord behind.

Gill slits lie on both sides of the pharynx at some stage or throughout life; in advanced chordates they disappear. Below the pharynx sits a special organ, the endostyle, which later turns into the thyroid gland. The chordate circulatory system is closed, haemoglobin is inside the red blood cells, and a portal system is present.

Vertebrates have two pairs of lateral limbs, held up by the endoskeleton. In the embryo there is a muscular, elastic tail behind the anus, and in many animals it disappears later. Chordate segmentation is limited to the body wall, brain and tail; it does not reach the coelom.

Turn the five cards and see what becomes of each in an advanced animal.

3. Three subphyla

Chordata has three subphyla: Urochordata, Cephalochordata and Vertebrata. The first two have no vertebral column, so together they are called Protochordata, the primitive chordates. Open the chart and see what hangs under each branch.

4. Urochordata: the sea squirts

Urochordata comes from Greek oura (tail) and chorda (cord). There are about 3,000 species, all marine, living in shallow water along coasts, singly or in colonies. They squirt water through a siphon, hence the name sea squirt. The adult has no notochord; it is found only in the larval tail. The adult is sessile and the larva a free swimmer. The body is covered by a tunic (test) made of cellulose.

It has three classes: Ascidiacea, Thaliacea and Larvacea. Ascidiacea has a swollen or tubular body, a permanent, thick, semitransparent covering and no tail in the adult. Examples: Ascidia mentula and Molgula tubifera.

Thaliacea look like a lemon or a barrel, with a thin transparent covering and a tailless adult. Examples: Salpa maxima and Doliolum rarum.

Larvacea are shaped like a curved tadpole, with a temporary, gelatin-like transparent covering. Their adult keeps its tail. Example: Oikopleura dioica.

Drag time from the swimming larva to the fixed adult. Watch where the notochord was, and when it is gone.

5. Cephalochordata and Vertebrata

Cephalochordata comes from Greek kephale (head) and chorda (cord). They live on sandy coastal bottoms and show the earliest, simplest form of every chordate feature. 33 species are known. The body is long, laterally compressed, transparent and pointed at both ends. In front, on the ventral side, sits an oral hood with oral cirri.

Here the notochord and nerve cord last for life and run from front to back. Many gill slits in the pharynx open into the atrium. Along both sides lie “>”-shaped myotome muscles. Example: Branchiostoma lanceolatum, better known as Amphioxus.

Vertebrata is the third chordate subphylum (Latin vertebratus = with a backbone). The brain lies inside a bony or cartilaginous cranium, so it is also called Craniata. A backbone of vertebrae takes the place of the notochord. The dorsal hollow nerve cord forms the brain and spinal cord; the vertebrae enclose the spinal cord and the skull protects the brain.

The pharynx has 5 to 15 pairs of gill slits, only in the embryo of advanced vertebrates. Paired lateral appendages, fins or limbs, are the organs of locomotion.

Now the names. Exams give an animal and ask for its class or subphylum.

6. Chordata versus Nonchordata

In Chordata the notochord is present at some time or throughout life, gill slits at some stage, and a tail early in life or throughout. In Nonchordata the notochord is never present; even where gills exist there are no gill slits, and there is no true tail.

The nerve cord and heart sit on opposite sides. In chordates the nerve cord is above the notochord, hollow and tubular, and the heart is ventral to the gut. In nonchordates the nervous system is solid and ventral; the heart may be absent, and if present it is dorsal to the gut. Haemoglobin is inside the red cells in chordates, and in nonchordates, where present, it is dissolved in the plasma.

7. Check yourself

Start with the first question.

The second is from the same phylum.

Pause and think once before answering the last one.

The whole note on seven cards.

Source: Biology 2nd Paper (Zoology), Classes 11-12 (NCTB), Chapter 1 “Animal Diversity and Classification”, the “Phylum Chordata and its Subphyla” part.