i68 



COMPAEATIVE ANATOiSIY. 



Brancliial Skeleton. 



§ 353. 



A ventral system of arclies is connected witli the most anterior 

 portion of tlie axial skeleton; and forms the organs of support for 

 that portion of the alimentary canal which functions as a respiratory 

 cavity. The number of arches, and the backward extension of this 

 apparatus, depends on the size of this respiratory cavity. We meet 

 with two very different types of these structures. 



The first type is found in the Acrania (Amphioxus). In this 

 framework there is a cartilaginous arch around the mouth — that is, 

 in its most anterior portion ; this arch is beset with cartilaginous 

 rods which are directed forwards. The rest of the apparatus is 

 formed of a homogeneous substance, which forms, as in Balano- 

 g'lossus (cf. § 112), a complicated lattice-work. The branchial bars 

 of either side are independent of those on the other ; that is to say, 

 they are not united along the ventral line. 



We cannot derive the second type, which obtains in the 

 Craniota, directly from this. In its earliest stage it is made up of 

 cartilaginous pieces only ; these do not form so large a number of 

 arches as exist in Amphioxus, and are, while completely symmetrical 

 as regards their arrangement, united ventrally by a copula. 



In the Cyclostomata the branchial skeleton is made up of com- 

 plicated cartilaginous bars, which are connected inferiorly with one 

 another, as well as with either side of the spinal column superiorly ; 

 omng to their superficial position they may be spoken of as forming 

 an external branchial framework. Very evident signs of this 

 are retained by the Selachii, but in them there is another, or 



internal, organ of sup- 

 port; and this is found in 

 all the rest of the Yerte- 

 brata. 



The various arches pre- 

 sent indications of their 

 primitive similarity ; this 

 disappears in consequence 

 of the gradual change in 

 their functional relation, 

 which is due to a divi- 

 sion of labour. We were 

 obliged to speak of some 

 of these arches in deal- 

 ing with the cranium; so 

 that now they need be but briefly considered. The first of them 

 surrounds the entrance to the alimentary canal, and is divided into 

 two pieces; one, superior, the palato-quadrate (Fig. 255, o), and the 

 other inferior, the primitive lower jaw (»). The succeeding pairs 



Fig. 255. Skull and branchial skeleton 

 of a Selaoliian (Diagrammatic), ab c Labial 

 cartilages. I Mandibular arch, o Upper, tc Lower 

 portion. 11 Hyoid arch. Ill — Till Branchial 

 arches. 



