Amphioxus, belonging to Cephalotaxus, has the characteristics of typical chordates, such as developed notochord, dorsal neural tube, pharyngeal branchial fissure and anal tail. There is only one class and one family in this subfamily, namely cephalochordae and branchiae, and amphioxus belongs to one of them. Amphioxus has no head, heart and bones, but it has organs or systems unique to higher animals, such as skin, bones and muscles, digestive and respiratory organs, circulatory system, nervous system and sensory organs, excretory and reproductive systems. Many lower animals have no heads, hearts and bones, but they can still survive.

First, the skin

Skin is thin and translucent, with epidermis and dermis.

Second, bones and muscles

The notochord is the supporting structure of amphioxus. Amphioxus has thick back muscles and thin abdomen. There are more than 60 pairs of muscles in the whole body, arranged according to the questions. & lt？ Serrated sarcomere, with the tip facing forward; The staggered arrangement of muscle segments on both sides is asymmetric, which is beneficial to the body swing of amphioxus.

Three, digestive and respiratory organs

Amphioxus passively eats. The vestibular wheel can stir the water into the mouth, and the sediment is kept out of the mouth. Its pharynx is particularly large, occupying almost 1/2 of the whole body length. The intestine is the rectum, which can secrete digestive juice. Once, the food mass mixed with digestive juice was stirred violently through it, making the digestion more thorough. Amphioxus has a large number of capillaries on the pharyngeal wall, gill slits are directly opened on the body surface, and the lymphatic sinus under the thin skin can directly absorb oxygen.

Fourth, the circulatory system.

This is a closed loop without a heart. The abdominal aorta and the base of each branchial artery located on the ventral surface of the digestive tract have the ability to beat and become the branchial heart.

Verb (abbreviation for verb) excretory and reproductive system

The excretory organ is the renal tubule, which is hermaphrodite and fertilized in seawater.

Six, the nervous system and sensory organs

The dorsal neural tube of amphioxus has almost no differentiation of brain and spinal cord. The anterior chamber is slightly enlarged, which is a brain bubble. The sensory organs are underdeveloped and have photoreceptors. There are sporadic sensory cells walking in the skin of the whole body, and there are many tentacles and limbal membrane tentacles, which can feel the chemical properties of water flow.