Cross references:  Amphioxus Hormones   Amphioxus Nervous System  
Deuterostomes   Chordates      Hemichordates     Protochordates   

Chordate (Wiki) 
(the source of this diagram) 

File:BranchiostomaLanceolatum PioM.svg

1 = bulge in spinal cord ("brain")
4 = post-anal tail
5 = anus
9 = space above pharynx
11 = pharynx
13 = oral cirri
14 = mouth opening
16 = light sensor
17 = nerves
19 = hepatic caecum (liver-like sack)

Note:  This diagram leaves out two very important structures: the frontal eye and the lamellar organ. 

Amphioxus (Wiki) 
"The lancelets (also known as amphioxus) are the modern representatives of the subphylum Cephalochordata, usually said to be the sister group of the craniates. They are usually found buried in sand in shallow parts of temperate or tropical seas.

They are an important object of study in zoology as they provide indications about the origins of the vertebrates. Lancelets serve as an intriguing comparison point for tracing how vertebrates have evolved and adapted. Although lancelets split from vertebrates more than 520 million years ago, their genomes hold clues about evolution, particularly how vertebrates have employed old genes for new functions.[2] They are regarded as similar to the archetypal vertebrate form.

Anatomy of a Branchiostoma lancelet
Anterior external anatomy
A: buccal cirri, B: wheel organ, C: velum, D: rostrum, E: notochord extending beyond nerve cord, F: nerve cord, G: Hatschek's pit, H: fin rays, I: gill bar, J: buccal cavity (vestibule)
Cross section through the pharyngeal region
A: fin box, B: nerve cord, C: notochord,
D: myomeres, E: epibranchial groove,
F: endostyle, G: secondary gill bar,
H: primary gill bar, I: pharyngeal cavity,
J: metapleural fold, K: epidermis.

Lancelets grow up to about 5 centimetres (2.0 in) long, reaching 7 centimetres (2.8 in) at the longest. They have a translucent, somewhat fish-like body, but without any paired fins or other limbs. A relatively poorly developed tail fin is present, but they are not especially good swimmers. While they do possess some cartilage-like material stiffening the gill slits, mouth, and tail, they have no true skeleton.[4]

In common with vertebrates, lancelets have a hollow nerve cord running along the back, pharyngeal slits and a tail that runs past the anus. Also like vertebrates, the muscles are arranged in blocks called myomeres.

The genome of the Florida lancelet (Branchiostoma floridae) has been sequenced.[7]


snowbio - Lancelet (fish-like chordate) 


Introduction to the Cephalochordata (Goog)
A brief introductory article available complete online. 
Note that cephalochordates have all the typical chordate features. The dorsal nerve cord is supported by a muscularized rod, or notochord. The pharynx is perforated by over 100 pharyngeal slits or "gill slits", which are used to strain food particles out of the water. The musculature of the body is divided up into V-shaped blocks, or myomeres, and there is a post-anal tail. All of these features are shared with vertebrates. On the other hand, cephalochordates lack features found in most or all true vertebrates: the brain is very small and poorly developed, sense organs are also poorly developed, and there are no true vertebrae.

Cephalochordata (Lancelets) (Goog)    
Long, informative article available online for free. 

Immunocytochemical Localization of Serotonin in Embryos, Larvae and Adults of the Lancelet, Branchiostoma Floridae (Goog) 

For discussion, see: 
Amphioxus Serotonin System  . 

Laboratory spawning and development of the Bahama lancelet, Asymmetron lucayanum 
Only abstract available online.