Fishes of the Western North Atlantic 5 



ity was found feeding chiefly on bacteria, with a few protozoa also. The intestines of some 

 of the specimens were filled with sand, showing that the oral tentacles do not always bar 

 entrance to inedible particles. 



Development. The larval development of the Lancelets has been the subject of 

 several major investigations and has been much discussed in relation to the problem of the 

 ancestry of the vertebrates. The process in the European Branchiostoma lanceolatum, 

 which may serve as representative of the group, is briefly as follows. 



Spawning takes place at sunset. The eggs are minute (o.i mm. in diameter) and float 

 freely in the water. Segmentation is not only complete but nearly equal and affords one of 

 the classic examples of endoderm formation by invagination. About twelve hours after 

 fertilization the embryo, now oval in shape and clothed externally with cilia, breaks out 

 from the vitelline membrane and swims near the surface by ciliary action. By_ about the 

 thirty-sixth hour the yolk is entirely absorbed} the mouth has appeared on the left-hand 

 side; the first gill opening has been formed in the midline, soon to shift to the right side, 

 however; and the anus has formed at the hinder end of the body a little to the left of the 

 midline. During subsequent larval development, which may occupy as much as three 

 months, the larvae live pelagically some distance below the surface of the sea, hanging 

 for the most part in a vertical position which is maintained by the action of the long cilia, or 

 flagellae, one of which is borne by each cell of the ectoderm. The larvae (Fig. i), which 



Figure i. Branchiostoma lanceolatum Pallas; larva, with 6 1 myotomes, after Franz, a anus, gi gills, i intestine. 

 nto mouth, n notochord. nc nerve cord. 



have a very characteristic appearance because of the swollen gill region in an otherwise 

 slender body, gradually assume the characters of the adult without any abrupt metamor- 

 phosis. The most striking of the external accompanying changes are in the numbers and 

 locations of the gill openings, and the formation of the atrium, of the atriopore and of the 

 adult mouth. The latter, at first on the left side and forming a most conspicuous feature of 

 the larva because of its enormous size, shifts to the midline and decreases in relative size 

 toward the end of larval life, while the preoral hood then develops above it. Additional 

 gill openings, up to the number of 14 or more, are formed successively along the mid- 

 ventral line, corresponding at first in number and location to the myotomes in that part of 



