68 



Evolution and Distribution of Fishes 



Fig. 3. Vertical section of young larva of Lamprey {Petromy- 

 zon) showing notochord, ch, mouth, 0, velum, i^, developing 

 thyroid gland, //, that is the probable homologue of the glandular 

 mid-oesophagus of Metanemerteans, and of the endostyle in primitive 

 vertebrates. 



Oesophagus. The small orifice of the gullet in higher 

 nemerteans leads into the oseophagus, which in some land 

 nemerteans shows division into three parts, an anterior 

 narrow tube, an enlarged highly glandular mid-area, and a 

 more constricted slightly glandular posterior part. But 

 the mid-area is not merely glandular, some if not all of its 

 cells are "very richly ciliated." It would thus exactly cor- 

 respond in requirements to a primitive and diffuse condition 

 of the endostyle in ascidians, to the hypopharyngeal groove 

 of Amph'wxus, and to the thyroid gland of fishes, as well 

 as of higher vertebrates. In the description by von Graff 

 (5^:430) and Dendy {35'- 92)) ^^ is not determined 

 whether the cells are in part glandular in part ciliate, or 

 as possibly may be the case are both conjoined. Burger's 

 descriptions {6: 198-99) and figures indicate that there are 

 distinct ciliate and glandular cells but either condition 

 would furnish the basis for gradual differentiation and re- 

 striction of the cells to the ventral surface of the oesopha- 

 gus, to in part a glandular In part a ciliate state, to the con- 

 densing of these into four longitudinal tracts, and thus to 

 formation of the endostyle or hypopharyngeal groove 

 (Fig. 4.gl) of the primitive chordate types Ascidia and 

 Amphioxus. 



Now while In the larva of Petromyzon It remains an 

 open groove, It later becomes closed, "highly complicated, 

 with paired anterior and posterior horns and a median 

 spiral portion," eventually in the adult "the organ is partly 



