460 SUB-PHVLUM 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 
larve. 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 swint. 
Form.—The body, between 14 and 2 in. in length, is 
pointed at both ends, as the names suggest. The living 
Fic. 248.—Lateral view of Amphzoxus.—After Ray Lankester. 
The notochord runs from tip to tip. : 
z., Tentacular cirri ; G., reproductive organs ; a.g., atriopore ; 
@., 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; (4) the atriopore in myotome thirty-six, giving exit to 
the water which enters by the mouth; (¢) 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 
