Amphioxus

Amphioxi, also known as lancelets, are small marine animals, seldom larger than 8 cm long, They resemble small, slender fish without eyes or definite heads.It has the basic chordate features: a notochord, gill slits, and a dorsal nerve cord. They have very basic digestive and circulatory systems, and are probably a representative of the most basic type of life form. Life for an organism is defined as being able to maintain homeostasis, be able to undergo metabolism, possess a capacity to grow, respond to stimuli, undergo reproduction, and be able to adapt to environmental changes.

http://www.britannica.com/EBchecked/topic/21580/amphioxus
http://en.wikipedia.org/wiki/Lancelet
http://en.wikipedia.org/wiki/Life