Fig. 89.—Diagrammatic transverse sections of three larvae of Amphioxus to show the development of the atrium. ao, Aorta; c, dermis; ch, notochord; d, intestine; f, connective tissue; fh, cavity of dorsal fin-ray; m, myotome; n, nerve-tube; p, atrium; sf, metapleural folds; sfh, lymph space in metapleural folds; si, sub-intestinal vein; sk, sheaths of notochord and nerve-tube; sl, sub-atrial ridge; sp, coelom. (From Korschelt and Heider, after Lankester and Willey.)
The mouth now becomes median and ventral, and is reduced in size, the oral hood (stomodaeum) is formed in front of it, the gill-slits become more numerous and vertically elongated, the endostyle forms along the floor of the pharynx, and the gonads grow as paired pouches from the body-wall. This brings the animal to the young adult condition, reached at a period of probably about three months after the fertilisation of the egg.
The development as a whole shows a very marked resemblance to that of the Tunicata (see p. [55]), but lends no support to the view that Amphioxus has degenerated from a higher group of the Vertebrata.
Classification of the Cephalochordata.
The known species of Amphioxus may be classified as follows[[116]]:—
Family Branchiostomatidae.
Genus 1. Branchiostoma (Costa, 1834).
Having biserial gonads and symmetrical metapleura.
B. lanceolatum (Pallas)—Myotomes 36 + 14 + 12, gonads 23-29 pairs: Mediterranean, N.W. Europe, Ceylon, E. of United States.
[B. belcheri, Gray—Myotomes 38 + 17 + 9: Torres Straits, Singapore, Borneo, Ceylon.