Bulletin ^7, United States National Museum. 



Class I. LEPTOCARDII. 



(THE LANCELETS.) 



Skeleton membrane-cartilaginous. Notochord persistent and extending 

 to the anterior end of the head, inclosed in a membranous sheath as is 

 the cord-like nervous axis above it. Heart a longitudinal tubular vessel 

 which gives off branchial vessels which unite in an aorta; end of the 

 nervous axis not dilated into a brain and not surrounded by a protective 

 capsule, or skull. Blood colorless. Respiratory cavity confluent with the 

 cavity of the abdomen ; gill slits in great number, the water being expelled 

 through an abdominal pore in front of the vent. Jaws none; the mouth a 

 longitudinal fissure, with cirri on each side. Body lanceolate in form, 

 more or less fish-like, and not enveloped in a tunic. Dorsal fin present, 

 low ; anal fin usually more or less developed. 



Small marine animals, highly interesting to the zoologist as exhibiting 

 the lowest degree of development of the vertebrate type. The class in- 

 cludes but the single order, AMPHIOXI or CIRROSTOMI. (A^rof, thin; 

 ictptiia, heart.) (Subclass LEPTOCARDII, Giinther, Cat., vm, 513-514.) 



Order A. AMPHIOXI. 



(THE ClRROSTOMES.) 



This order is equivalent to the family BRANCHIOSTOMID^E. (a//ot, both ; 

 oftff, sharp : Cirrostomi is from cirrus, a lock of hair; ar6//a, mouth.) (CiR- 

 ROSTOMI, Giinther, Cat., vm, 513-514.) 



Family I. BRANCHIOSTOMID^E .* 

 (THE LANCELETS.) 



Body elongate, lanceolate, compressed, naked, colorless; the fins repre- 

 sented by a low fold extending along the back, with usually a rudimentary 

 fold below which passes by the vent to the abdominal pore. Mouth in- 

 ferior, appearing as a longitudinal fissure, surrounded by conspicuous, 

 rather stiff, cirri. Eye rudimentary. Liver reduced to a blind sac of the 

 simple intestine. Small, translucent creatures found imbedded in the 

 sand on warm coasts throughout the world. Eight species are now recog- 

 nized, referable to 2 or 3 genera. They are all very similar in appear- 

 ance and habits, and the numbers of the muscular impressions furnish 

 the only characters thus far known by which the species can be distin- 

 guished, t (CIRROSTOMI, Giinther, Cat., vm, 513-514.) 



a. Gonads (reproductive structures) present on both sides of the median line ; anal fin present, 



with traces of fin rays ; no caudal process. BRANCHIOSTOMA, 1. 



aa. Gonads present on the right Bide only ; anal fin without fin rays or successive fin-ray 



chambers ; a long caudal process, or tail, about as long as the head. ASYMMETRON, 2. 



* In the arrangement of the genera and species of this group we have been guided by the con- 

 clusions of Dr. E. A. Andrews, in his recent paper, "An undescribed Acraniate : Asymmetron 

 lucai/anum." (Studies Biol. Lab. Johns Hopkins University, v, No. 4, 213-247, 1893.) 



fSee Giinther, Rept. Zool. Voyage H. M, S. Alert, 1884, where the value of this character is 

 first fully recognized. 



