Occasional Papers of the Muscimi of Zoology 3 



III 



In the construction of the following key to the families and 

 genera of the lancelets use has been made only of the more 

 obviously significant and better confirmed characters recorded 

 by various students of these primitive chordate animals. The 

 classification expressed in this key is intended to indicate, as 

 exactly as possible, the interrelationships of the groups. 



Kev to the families and aencra of lancelets 

 a^. Mouth a sinistral slit devoid 01 oral cirri; no closed atrial cham- 

 ber; gill-slits in an unpaired medioventral series; pharynx divided 

 into an upper pars nutritoria and a ventral pars rcspiratoria 



(Amphioxididae) i. Amphioxides^ 



a-. Mouth nearly median, with oral cirri ; a closed atrial chamber ; 

 gill-slits paired and lateral ; pharynx undivided. 

 b"^. Gonad pouches developed on both epipleura; metapleura of 

 each side terminating just behind atriopore (Branchiostom- 

 atidae). 



fi. No long rostral process 2. Branchiostoma 



C-. A long process, containing the anterior end of the noto- 



chord 3. Dolichorhamphus 



b'-. Gonad pouches developed only on right epipleur ; right meta- 

 pleur confluent with preanal fin (Epigonichthyidae). 

 d^. No urostyloid process ; oral cirri comprising a single series, 

 united one to the other' by a uniformly low intertentacular 



membrane 4. Epigonichthys 



d'-. A long urostyloid process; oral cirri comprising a lateral 

 group united by low membranes, and a ventral group 

 joined by a very high membrane 5. Asymmetron 



Subphylum Cephalochorda 

 Class Leptocardii 

 Order Amphioxi 



Family Amphioxididae Gill 

 Abundant evidence that the species referred to this group 

 are merely larval forms of other lancelets, perhaps Asym- 



1 The species which have been placed in this genus and family are 

 probably larval forms of other lancelets (see following list). 



