6 Memoir Sears Foundation for Marine Research 



the body but later losing this relationship. Of these primary gill openings, only the second 

 to ninth persist, however. 



After the formation of the primary series of gill openings the number of segments 

 increases at the posterior end of the body, the final number being attained early in larval 

 life. In the meantime the embryonic tail fin, a simple ridge of columnar ectoderm cells, 

 is replaced by the adult fin; this forms as an ectodermal fold, enclosing serial expansions of 

 the body cavity, the ray chambers; the fin rays develop as columnar outgrowths of meso- 

 derm upward into these chambers. A secondary series of gill openings, eight or nine in 

 number, appear on the right side of the body, dorsal to the primary series; and each mem- 

 ber of each set, except the first, becomes U-shaped and then entirely subdivided by a dorso- 

 ventral bar. The primary series of gill openings then shift to the left side of the body, so 

 that from then on the larva is bilaterally symmetrical so far as the location of its gills is 

 concerned. 



The metapleural ridges first appear in larvae with eight to ten gill openings of the 

 second series. The atrial cavity results from the union of the median sides of these ridges, 

 commencing posteriorly and progressing anteriorly. The canal so enclosed expands later- 

 ally in the pharyngeal region to the dimensions of the atrium of the adult, while it con- 

 tinues open posteriorly as the atriopore. During the formation of the metapleura the larva 

 abandons its pelagic habit and comes to lie on one side or the other on the bottom. By the 

 time the mouth has moved to the median position the oral hood has formed and the gills 

 have assumed the final symmetrical arrangement. The little Lancelet, now resembling the 

 adult in general appearance, buries itself in the sand; the only further change is the forma- 

 tion of pairs of tertiary gill openings, a process that continues throughout the life of the 

 individual. The curious asymmetry of the larval Lancelet has been much discussed, but in 

 our opinion none of the explanations which have been offered for it is adequate. 



Gonads are formed in the second or third year, and the oldest noted among a large 

 collection of Branchiostoma belcheri was four years old.^° 



Relation to Man. Lancelets are neither large enough or numerous enough to be of 

 any commercial value anywhere in the western Atlantic, except as subjects for biological 

 investigation; nor are they ever likely to be. However, near Amoy in southern China there 

 has long been a fishery for Lancelets. Recently this employed about four hundred men in 

 two hundred boats who fished with shovel- or scoop-shaped dredges from two to four 

 hours each day on the ebb tide from August until April. This fishing ground is only about 

 six miles long and less than one mile wide, but it has been estimated that the annual catch 

 is in the neighborhood of 35 tons, or more than one billion Lancelets. Some of these are 

 consumed in the near vicinity, while others are dried and shipped to Java and Singapore." 

 Lancelets are also used occasionally as food in Naples and Sicily.'^ 



10. See Wells (Science, N.S. 64, 1926: 188) for Branchiostoma caribaeum from Florida; Chin (Philip. J. Sci., 75, 

 1941 : 400) for B. belcheri from Amoy, China. 



1 1. For more detailed accounts, from which the foregoing is condensed, see Light (Science, N.S. 5^, 1923 : 57) *nd 

 Chin (Philip. J. Sci., t$, 1941 : 369). 



12. Franz, in Grimpe and Wagler, Tierwelt N- u. Ostsee, Lief 8, b, 1927 : 44. 



