The Chordate Phylum is currently the highest phyla in the animal kingdom, with three subphyla under it: the subphylum Tails, the subphylum Cephalopoda, and the vertebrate subphylum. Today I will talk to you about tail cord animals.
This "tail cord" is not the "obscenity" you think. Tails are found in all oceans of the world, about 2,000 species, and there are about 14 species known in China. Individual or group life, most of the tailless species of juveniles live freely, adult camp fixed life; a small number of lifelong tailed species camp floating free swimming life. Its representative species is the sea squirt.
Tail cord animal Red Sea Squirt
The sea squirt has an oblong oval shape, and the outermost layer of the body is encapsulated, and inside the infigus is a soft mantle membrane. There are two openings at the top, the top opening is the inlet pipe hole, and the side opening is the outlet pipe hole. It is fixed by the base of the body on the rocks, shells or bottom of the ship. Crowds. The flow of water through the body of the sea squirt is important for the survival and protection of the sea squirt. The sea squirt has no endoskeleton and relies on the current of water to generate internal pressure to support the body, while the water flow is necessary for the metabolism of the sea squirt.
Tails have been described as early as 2,000 years ago, but they have long been classified as invertebrates and suffered injustices, but real gold is not afraid of fire! In 1866, the Russian embryologist Kovalevsky systematically studied the embryonic development of sea squirts and established their taxonomic status as lower chordates. Why is this happening?
<h1 class="pgc-h-arrow-right" data-track="11" > larvae and metamorphosis</h1>
The ontogenesis of sea squirts is a process of metamorphosis. The fertilized eggs of the sea squirt undergo embryonic development to form tadpoles-like, free-swimming larvae, with a body length of only a few mm and a muscular lateral flattened tail. Juveniles have three characteristics of chordates: a typical chord in the long tail (hence the name Caudal cord); a hollow dorsal neural tube on the back of the chord, the anterior end of the neural tube expands into a brain vesicle, with eye points and balancers; the front end of the digestive tube differentiates into a pharynx, and there is a gill fissure on the pharyngeal wall. In addition, the tail is behind the anus, and the heart is located on the ventral surface. These are typical chordate features!
The tadpole-shaped juveniles of the sea squirt possess the three characteristics of chordates
But the good times are not long! After a few hours to 1 day of free swimming life, the larvae begin to find a suitable habitat with the help of simple eye and sensory cells, attach to an object with the attachment protrusions at the front of the body, and begin the process of metamorphosis. During metamorphosis, it loses some of the features it should have as a chordate: its tail, along with the spinal cord, gradually atrophied and disappears; the dorsal neural tube degenerates into ganglia; and the sensory organs disappear. However, due to the enlargement of the pharynx, the number of gill fissures on it is greatly increased.
The adhesion protrusion grows rapidly on the back and the rest of the plant grows slowly, causing its internal organs to rotate about 90°. The body wall then secretes anocysin to form a sac, and the sea squirt changes from a free swimming life of the larvae to a solid life. This more simplistic metamorphosis from juvenile to adult structure is called retrograde metamorphosis or degenerative metamorphosis. After retrograde metamorphosis, the sea squirt adults lost the chord and dorsal nerve tubes that are the three characteristics of the chordate! That's why it's mistaken for an invertebrate! Fortunately, sea squirt larvae possess all three characteristics of chordates at the same time, providing irrefutable empirical evidence for the classification of tail cords into chordates!