116 LECTURE V. 



and serve to support the membrane which is developed to form a 

 great receptacle on each side of the head of that singular fish. 



Branchial Arches. 



Certain bony arches, which appertain to tlie system of the visceral 

 skeleton, succeed the hyoidean arch, with the keystone of which they 

 are more or less closely connected. Six of these arches are pri- 

 marily developed, and five usually retained ; the first four of these 

 supporting the gills {Jig. 39. 4", 47), the fifth {ib. 47') beset with teeth 

 and guarding the opening of the gullet : this latter is termed the pha- 

 ryngeal arch, the rest the branchial arches. 



The lower extremities of these arches adhere to the sides of a me- 

 dian chain of ossicles, which is continued from the posterior angle of 

 the basi-hyal, or from above the uro-hyal, when this is ossified : the 

 arches curve as they ascend ; and their upper extremities, which are 

 usually distinct pieces, bend inwards and almost meet beneath the 

 base of the cranium, to which they are attached by ligamentous 

 and cellular tissue. 



The inferior median symmetrical piece is commonly divided into 

 three ossicles, (the basi-branchials, ib. 45), following each other in a 

 linear series along the median line : the first rests upon the uro-hyal, 

 when this is present ; or it is attached to the posterior interspace of 

 the cerato-hyals : the second gives attachment to the first pair of 

 branchial arches, and the third to the second pair of arches : the third 

 pair of arches is attached to the extremity of the second pair and to a 

 ligament continued from the third basi-branchial : the fourth pair of 

 arches adheres to the same ligament in the angle of the third pair : 

 and the pharyngeal arches, forming the fifth pair, are attached to the 

 angle of the fourth. 



Each branchial arch, independently of the basal key-bones, consists 

 of three or four pieces, enjoying a certain elastic, flexible movement 

 on each other. The first three arches consist each of a short piece 

 below, the hypo-branchial {Jig. 39. 46), which, in the Halibut, sends a 

 ridge or process downwards and inwards beneath the basi-branchials: 

 next, of a long bent portion, the cerato-branchial {ib. 47), grooved on 

 its outer convex side ; usually supporting dentigerous processes, 

 tubercles, or fine plates on its concave side : and, above, of a shorter, 

 similarly formed piece, bent inwards and forwards, the epi-branchial 

 {ib. 48). To the epi-branchial of the second and third arches is com- 

 monly attached a shorter and broader bone beset with teeth, the 

 pharyngo-branchial. 



The fourth arch consists of the cerato-branchial, the epi-branchial, 

 and the phai-yngo-branchial pieces. The fifth arch {ib. 47') usually 



