Bulletin ^7, Utiited States National Museum. 



Class I. LEPTOCARDII. 



(The Lancelets.) 



Skeleton iiiembrano-cartilaginous. Notocliord persistent and extending 

 to the anterior end of the bead, inclosed in a membranous sbeatb as is 

 tlio 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. Eespiratory 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, Ami'HIOXI or Cirrostomi. (^fTrrof, thin; 

 Kapdia, heart.) (Subclass LErrocARDii, Giinther, Cat., viii, 513-514.) 



Order A. AMPHIOXI. 

 (The CiRROSTOMES.) 



This order is equivalent to the family Branchiostomid^. {n/nol, both ; 

 o^i'r, sharp : Cirrotitomi is from cirms, a lock of hair; oTufxti, mouth.) (CiK- 

 KOSTOMi, Giinther, Cat., viii, 513-514.) 



Family 1. BRANCHIOSTOMID.^.* 



(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 pas.ses by the vent to the abdominal pore. Mouth in- 

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

 rather stifi", 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, Gunther, Cat., vni, 513-514.) 



u. Gonads (reproductive structures) present on both sides of the lucdiau line ; anal tin present, 



with traces of fin raj-s ; no caudal process. Branchiostoma, 1. 



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



cbamhers ; a long caudal process, or tail, about as long as the head. Asvmmetron, 2. 



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

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

 luounuwii,. (Studies Biol. I.ab. .lolms Ho|ikius University, v. No. 4, 213-247 189.3.) 



fSee Gunther, Kept. Zoul. Voyage U. M. S. Alert, lb84, where the value of this character is 

 first fully recognized. 



