Cephalocords are represented by animals known as amphioxes, which comprise about thirty species, all living in the marine environment. The word amphioxus derives from the fact that these animals have tapered bodies at two ends (amfi = two).
Amphioxes are small animals, measuring up to 8 centimeters in length. They have a fish-like body and live half-buried in the sand in clean, calm water, keeping only the anterior part of the body out of the substrate.
Although they spend most of their time buried, they can actively swim in the water for short periods of time. Amphioxus swimming is similar to that seen in fish: it results from the contraction of myotomes, muscle blocks arranged serially along the body. The alternating contraction of these muscles from one side and the other of the body promotes a lateral movement that propels the animal forward.
These animals have fins, but, unlike those found in fish, they are formed only by folds of skin, with no supporting skeletal elements within them; they only have connective tissue reinforcement.
Like the urocordates, the gill slits of the cephalocords are well developed, indicating the filtering habit of these animals. In either case, these slits do not open directly out of the body but into a cavity called the atrium.
In cephalocords the eyelashes of gill slits or pharyngans promote water ingress and egress through a special pore called an atriopore. They get the oxygen and food they need through this water circulation.
The filtered food particles are carried to the endostyle and from it to other parts of the digestive tract. Unusable remains are eliminated through the anus. In these animals the anus opens directly out of the body and not into the atrial cavity, as in the urocordates. In the atrium of the cephalocordates open the gonads.
O circulatory system It is formed only by vessels, some contractile, responsible for the propulsion of the blood. They have no heart.
Amphioxes are animals of separate sexes with external fertilization. They go through a plantar larval stage, after which they settle on the substrate and undergo metamorphosis, giving rise to the adult.
O nervous system of the cephalocordates, is quite simplified, being formed by a dorsal nerve cord, which presents a dilation in the anterior region called the cerebral vesicle.