The Leptocardii, or Lancelets 165 
their actual common ancestry with the fishes, they must ap- 
proach near to these in many ways. Their simplicity is largely 
primitive, not, as in the Tunicates, the result of subsequent 
degradation. 
The lancelets, less than a dozen species in all, constitute a 
single family, Branchiostomide. The principal genus, Branchi- 
ostoma, is usually called Amphioxus by anatomists. But while 
Fig. 115.—California Lancelet, Branchiostoma californiense Gill. 
(From San Diego.) 
the name Ampluioxus, like lancelet, is convenient in vernacular 
use, it has no standing in systematic nomenclature. The name 
Branchiostoma was given to lancelets from Naples in 1834, by 
Costa, while that of Amphioxus, given to specimens from Corn- 
wall, dates from Yarrell’s work on the British fishes in 1836. 
The name Amphioxus may be pleasanter or shorter or more 
familiar or more correctly descriptive than Branchiostoma, but 
if so the fact cannot be considered in science as affecting the 
duty of priority. 
The name Acraniata (without skull) is often used for the 
lower Chordates taken collectively, and it is sometimes applied 
to the lancelets alone. It refers to those chordate forms which 
have no skull nor brain, as distinguished from the Cramzota, 
or forms with a distinct brain having a bony or cartilaginous 
capsule for its protection. 
Origin of Lancelets.—It is doubtless true, as Dr. Willey sug- 
gests, that the Vertebrates became separated from their worm- 
like ancestry through ‘“‘the concentration of the central nervous 
system along the dorsal side of the body and its conversion 
