CYCLOSTOMATA. 



I O 



forming the line of insertion of the velum appears to me to represent the 

 mandibular arch. The grounds for this view are the following : 



nu.v 



f>n 



fit 



r.uo 



THROUGH THE HEAD OF A LARVA OK 



FIG. 43. DIAGRAMMATIC VERTICAL 



PETROM.YZON. 



The larva had been hatched three days, and was 4 - 8 mm. in length. The optic and 

 auditory vesicles are supposed to be seen through the tissues. The letter tv pointing 

 to the base of the velum is where Scott believes the hyomandibular cleft to be situated. 



c.Ii. cerebral hemisphere; tli. optic tlialamus ; in. iufundihulum; p. pineal gland; 

 i/ib. mid-brain; cb. cerebellum; uul. medulla oblongata; mi.r. auditory vesicle ; op. 

 optic vesicle; ol. olfactory pit; tn. mouth; br.c. branchial pouches; t]i. thyroid 

 involution; r.ao. ventral aorta ; lit. ventricle of heart ; cli. notochord. 



(1) The structure in question has r,<-<t<:tly the position usually occupied 

 by the mandibular arch. 



(2) There is present in late larvae (about 20 days after hatching) an 

 arterial vessel, continued from the ventral prolongation of the bulbus 

 arteriosus along the insertion of the velum towards the dorsal aorta, 

 which has the relations of a true branchial artery. 



On the ventral aspect of the branchial region is placed a sack 

 (figs. 42, It, and 43, th], which extends from the fruut end of the branchial 

 region to the fourth cleft. At first it constitutes a groove opening 

 into the throat above (fig. 44), but soon the opening becomes narrowed 

 to a pore placed between the second and third <,f the permanent 

 branchial pouches (fig. 43, tli). In Ammoccetes 1 the simple tube 

 becomes divided, and assumes a very complicated form, though still 

 retaining its opening into the branchial region of the throat. In the 

 adult it forms a glandular mass underneath the branchial region of 

 the throat equivalent to the thyroid gland of higher Vertebrates. 



somewhat scanty, but I have as yet. very likely owing to the imperfection of im- 

 material, been unable to find Scott's hyomandibular pouch either in my sections or 

 surface-views. Huxley describes this pouch as present in the form of a cleft in later 

 stages ; I have failed to find his cleft also. The vessel interpreted below as the 

 branchial artery of the mandibular arch was only imperfectly investigated by me, and 

 I was not sure of my interpretations about it. Scott however informs me by letter that 

 it is undoubtedly present. 



1 Schneider (No. 85) states that in the full-grown Arnmocretes the opening is situated 

 between the third and fourth pouches. This is certainly not true for the young larva. 



