Evolution of Fishes from Invertebrates 69 



end.l.-. 



Fig. 4a. 



Fig. 4b. 



Fig. 4a. Transverse section of anterior ventral part of ali- 

 mentary canal — or branchial sac — of Ascidia mentula, showing cili- 

 ated depression or endostyle (end . i) with four longitudinal patches 

 of glandular mucus secreting cells (gl). Below is a blood sinus 

 (v. bl. s). (After Herdman). 



Fig. 4b. Like section of Amphioxus showing endostyle (e) 

 with four glandular tracts (gl) that secret mucus; m.b.a, blood ves- 

 sel; sk, skeletal plates. (After Lankester). 



absorbed and partly divided up Into a series of glandular 

 follicles, and eventually forms the thyroid body."' As de- 

 scribed by Newton Parker ( J2 : 173) in Protopterus there 

 is a close resemblance, between the thyroid In it, and the 

 meso-oesophagus In Metanemerteans; and this gives added 

 value to Kerr's contention that the Dipnoi are an ancient 

 and likewise a primitive group, which retains many simple 

 ancestral conditions. The descriptions and figures of 

 Dendy suggest four paired glandular ciliated and columnar- 

 celled lobes that occupy the mid-ventral head region In 

 Geonemertes australis. A more detailed study of the meso- 

 oesophagus In freshwater and In land nemerteans is how- 

 ever highly desirable, as thus presenting the probable dif- 

 fuse beginnings of the vertebrate thyroid organ. 



VI. Respiratory system. 



At this juncture It may be appropriate to study the 

 primitive beginnings of the respiratory system. The writer 



