Fis/ics of tJic Western North Atlantic j 



Fni/iilit's. The order includes two well defined families, Branchiostomidae and 

 Epigonichtlnidae, separated as indicated in the following key. A third assemblage of pe- 

 lagic forms, usually grouped together as the genus Amp/iio.xides, have sometimes been 

 classed as a third family, Amphioxididae. But their chief distinguishing characters — mouth 

 on the left side, atrial chamber unclosed and gill slits in an unpaired medio-ventral series — 

 are those of larval Lancelets in general at an early stage of development (p. 23), and it 

 now seems established in fact that they are larvae that have continued their pelagic exist- 

 ence for one reason or another until much larger and much further advanced in develop- 

 ment than is usually the case, rather than taking to the bottom at a smaller size, as most of 

 them do.*' In fact, we think it is likely that these Amphioxides larvae ne\'er do descend to 

 the bottom once they are carried out over deep water, but that they simply continue to exist 

 for an indefinite period as they are swept along with the currents, finally perishing without 

 producing offspring. On the other hand, it has been suggested that their existence may 

 provide a means for the dissemination of the species. Up to the present time, none of them 

 has been positively connected with any particular parent species. 



Key to Families'* 



I a. Mouth nearly median, with oral cirri; closed atrial chamber and atriopore; a series of 

 gill clefts on either side. 



2a. Series of gonads developed on each side; both metapleura terminating close be- 

 hind atriopore. Branchiostomidae, p. 7. 

 2b. Gonads developed on right side only; the right metapleuron continuous with 

 ventral fin, the left-hand metapleuron terminating behind atriopore. 



Epigonichthyidae, p. 18. 



lb. Mouth on left side without oral cirri; no closed atrial chamber; gill clefts in a single 



series along ventral side. Amphioxididae, p. 23.'^ 



Family BRANCHIOSTOMIDAE 



Description. Mouth nearly in midline, surrounded by oral cirri; tentacles with lateral 

 sensory papillae, giving them a toothed appearance; closed atrial chamber; a series of gill 

 slits on each side; gonad pouches developed on both left and right sides; both metapleura 

 terminate close behind atriopore, including between their posterior ends the anterior end 

 of the ventral fin; rostral fin continuous with right side of oral hood, but not with left side; 

 posteriorly the median fin is expanded both dorsally and ventrally in lancet form as a 

 distinct caudal fin, with its ventral lobe lying to the right of anus; ventral fin-ray cham- 

 bers, except for the more anterior and more posterior, each contain a pair of fin rays in most 

 species, although perhaps only a single fin ray in some;"^ but dorsal fin-ray chambers con- 



13. For a recent discussion, see Goldschmidt (Biol. Bull. Wood's Hole, 6^, 1933: 321). 



14. Ainpliioxididae included to facilitate identification. 15. See above discussion of these. 



16. Willey (Quart. J. micr. Sci., 44, 1891: 270) stated that in his Dolic/ior/iync/ins injicus the ventral fin rays arc 

 sintj'le; but they appear as paired in his illustration. 



