CHAPTER ONE 



Lancelets 



BY 

 HENRY B. BIGELOW and ISABEL PEREZ FARFANTE 



ACKNOWLEDGMENTS 



JVe are indebted to Thomas Barbour and Leonard P. Schultz for putting the 

 Lancelet collections of the Museum of Comparative Zoology and of the United 

 States National Museum at our disposal for study. Also, hearty thanks are 

 due to Gerardo Canet for preparing all the original drawings included here. 



GENERAL DISCUSSION 



The Lancelets of the western Atlantic Ocean are included in the present volume for con- 

 venience, following the precedent established in existing manuals of the fishes of various 

 parts of the world. Actually they are not fishes at all, although fish-like in appearance, but 

 belong to a separate subphylum (Cephalochordata) of the Chordata, since they are much 

 simpler in structure than are any of the true vertebrates of the subphylum Euchordata, 

 or Vertebrata. 



Class LEPTOCARDII 



The notochord, extending the entire length of the body and persisting throughout 

 life, is surrounded by a resistant sheath, this notochord and sheath forming a firm but 

 flexible supporting structure. But there is neither protective skeleton nor cranium for the 

 anterior part of the neural tube, no bony structures of any sort, and no jaws. The pharynx 

 in the adult is surrounded by an atrial chamber, formed by the outgrowth and coalescence 

 of two ridges (the metapleura) of the body wall; the pharynx opens into the atrium by a 

 double series of gill slits, the number of which continues to increase throughout life; pos- 

 teriorly, the atrial cavity opens to the exterior by a small aperture, the atriopore. The dorsal 

 nerve tube terminates anteriorly some distance behind the anterior end of the notochord; 

 it is much compressed laterally, and the only suggestion of a brain is that its axial canal 

 widens anteriorly into a cerebral vesicle. The nerves given oflF by the neural tube (except 



