THE MAIN FACTS CONCERNING THE LANCELET. 31 



mouth. In both is the same lowly development of sense organs, and 

 want of any defined cephalic or head region. 



The points of divergence are essentially the presence in the 

 Lancelet, of a notochord, of an elongated central nerve-mass, and of 

 a higher type of blood system. 



Finally it is noteworthy that a series of minute tubules, probably 

 excretory, open into the upper portion of the atrial cavity on either 

 side of the pharynx. There can be but little doubt that these 

 represent the paired excretory tubules (nephridia), so characteristic 

 of the worms. Hence, as no kidneys of any vertebrate pattern are 

 present, the Lancelet exhibits in the structure of its renal organs, 

 the most unmistakable of its many plebeian characteristics. 



EXPLANATION OF FIGS. 13-16, PLATE IV. 



Amphioxus lanceolatus (Yarrell). 



Fig. 13. View of a young specimen iVin long ; from the left side. 

 Fig. 14. Diagram of a transverse section of same through pharyngeal 



region. 

 Fig. 15. Diagram of a transverse section through the anterior end 



of the body of a typical section, e.g. A. mentula. 

 Fig. 16. Diagrams showing the mode of formation of a double gill 



slit from a single. 



Lettering the same in all figures, viz. : a p. atrial pore ; a. anus ; 

 b t. buccal tentacles ; c. cuticle ; df. dorsal fin ; e. eye ; end. endostyle ; 

 ep [/. epibranchial groove ; g a. gill arches ; i. intestine ; L liver ; 

 m. muscles ; n. nerve system ; nt. riotochord ; ph. pharynx ; p c. 

 pigment cells ; sp c. spinal cord ; vf. ventral fin. 



