THE LANGELET {AmpUoxus, syn. Branchiostoma larweolatus) 



Amphioxus is a small translucent animal, fairly abundant in the shallow 

 coastal waters around the Mediterranean and other seas. A species, Amphioxus 

 (Branchiostoma) capensis, is fairly common in South African waters, particularly 

 in False Bay. 



External Features. 



The body is elongate, laterally compressed and pointed at both ends. 

 Each side is marked by a series of <-shaped lines representing the connective 

 tissue septa (myocommas) between the ^-shaped lateral muscle segments or 

 myotomes. Along the dorsal surface there is a median fold of skin forming the 

 dorsal fin. This fold is extended round the tail region as the caudal fin, and is 

 continued ventrally through the posterior third of the body as the ventral or 

 anal fin. 



Anteriorly the head region overhangs the oral hood, the edge of which bears 



Dorsal fin 



Fin rays 



Oral cirri 



Posi^on of / 

 Atriopore ; 



Anus c^udid fin 



Metapleural fold g^v^j^ 



Ventral fin 

 Fig. 48. — The Lanoelet (Amphioxus laneeolatus) viewed from the left side, x 3. 



numerous ciliated processes, the buccal cirri. In the centre of the oral hood is 

 the buccal opening. On the ventral surface of the anterior two-thirds of the 

 body are the two lateral metapleural folds, extending from the oral hood to the 

 anterior end of the ventral or anal fin. 



The atrial aperture or atriopore is a median opening on the ventral surface 

 at the junction of the ventral fin and the metapleural folds. 



The anus is also on the ventral surface, about midway between the atrial 

 aperture and the end of the body, slightly to the left of the middle line. 



The gonads may be seen through the semi-transparent wall of the atrium. 



Make drawings of the animal viewed (a) from the left side and (6) from 

 beneath to show the above features. 



Internal Structure. 



Place under water a specimen that has been macerated in 20 per cent, nitric 

 acid for about a day, and fix it down on its left side by two pins crossed over it 

 at each end. Carefully remove the entire body-wall of the right side, stripping 

 off the muscle segments from in front backwards, with a scalpel or a pair of 



142 



