The atrium is an ingrowth of the external space between the two ventral metapleural or atrial folds (Figs. 88 and 89), paired lateral ridges of the body-wall, and so is lined by ectoderm. This ingrowth is shut off from the exterior by the {136}growth towards each other of sub-atrial ridges on the inner sides of the metapleural folds (see Fig. 89, A, _sl_), and then becomes greatly enlarged by the increased relative growth of the ventro-lateral part of the body-wall (Fig. 89, B, C). The posterior opening between the metapleural folds remains as the atriopore (Fig. 88, C, _ap_); while the anterior end (Fig. 88) also remains open for some time, but eventually closes. As the metapleural folds lie outside the gill-slits (Fig. 88, A) when these folds close in (B and C), it comes about that the gill-slits which formerly opened freely to the exterior now open into the cavity of the atrium (compare Figs. 87 and 88). [Illustration: 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 {137}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. [_B. nakagawae_, Jord. and S.—Myotomes 37 + 16 + 11: Japan. [_B. caribbaeum_, Sundevall—Myotomes 37 + 14 + 9: West Indies, Atlantic, N. and S. America. _B. capense_, Gilchrist—Myotomes 47 + 19 + 9: S. Africa. _B. californiense_, J. G. Cooper—Myotomes 45 + 17 + 9: California. _B_. (_Dolichorhynchus_) _indicum_ (Willey)—Myotomes 42 + 14 + 15: India and Ceylon. (?) _B. elongatum_, Sundevall—Myotomes 49 + 18 + 12: Peru.