CYCLOSTOMATA. 



75 



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

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



Fig. 43. Diagkammattc vehtical skction through the hkau of a larva of 



Petkomyzon. 

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

 c.h. cerebral hemisphere ; «/;. optic thalamus; i?i. infundibulum; pn. pineal gland; 

 )/(/). mid-brain; cb. cerebellum; iiul. medulla oblongata; auA\ auditory vesicle ; op. 

 optic vesicle; ol. olfactory pit; m. mouth; br.c. branchial pouches; th. thyroid 

 involution; v.ao. ventral aorta ; ht. ventricle of heart ; ch. notochord. 



(1) The structure in question has exactly the position usually occupied \ 



by the mandibular ai'ch. 



(2) There is present i\\ late larvai (about 20 days after hatching) an f 

 arterial vessel, continiied fi*om 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 ai'tery. 



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

 (figs. 42, li, and 43, th), which extends from the front 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 of the 'permanent 

 branchial pouches (fig. 43, th). In Ammocoetes' 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 my 

 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 fahed 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. 



^ Schneider (Mo. 85) states that in the full-grown Ammocoetes the opening is situated 

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



