5i6 



CEPHALOCHORDA 



Amphioxus lanceolatus, the best-known species 



Mode of life. — The lancelets are fond of lying in the 

 sand in water about two fathoms deep, with only the fringed 

 aperture of the mouth projecting. They feed on diatoms 

 and other small organisms, which are sucked into the 

 mouth. At times, especially in the evening, the adults start 

 up and swim about, but they are never so active as the 

 larvae. The early embryo is pelagic. It is of interest to 

 note that along with lancelets, specimens of the Annelid 

 Ophelia are often obtained ; they closely resemble lancelets, 

 not only in shape and size, but also in the way they burrow 

 and swim. 



Form. — The body, between ij and 2 in. in length, is 

 pointed at both ends, as the names suggest. The living 



G. 



Fig. 295 



-Lateral view of Amphioxus. — After Ray Lankester. 



The notochord runs from tip to tip. 

 t., Tentacular cirri; G., reproductive organs; a.p., atriopore ; 

 a., position of anus ; 40 and 62, indicate number of myotomes. 



animal is translucent, with a faint flesh colour, and is much 

 plumper than a spirit specimen. The muscles are arranged 

 in sixty-two segments or myotomes. There are three un- 

 paired apertures — {a) the median, ventral, pre-oral hood 

 overarching the true mouth, and fringed with tentacle-like 

 cirri ; {b) the atriopore in myotome thirty-six, giving exit to 

 the water which enters by the mouth ; ic) the anus, ventral 

 and slightly to the left, behind the atriopore, but some 

 distance from the posterior end of the body. Along the 

 back there is a median fin, which is continued around the 

 tail, and along the ventral surface as far as the atriopore. 

 In front of this region the ventral surface is flattened, and 

 fringed on either side by a slight fin-like " metapleural " 

 fold. These folds are continuations downwards of the walls 

 of the atrial or branchial chamber, which extends from 



