4G3 



COMPAEATIVE ANATOMY. 



Branchial Skeleton. 



§ 353. 



A ventral system of arches is connected with the most anterior 

 portion of the 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- 

 glossus (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 lai'ge 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 ; 

 owing 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 Verte- 

 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 

 obliged to speak of 

 of these arches in 

 with the 



Fig - . 255. Skull and branchial skeleton 

 of a Selachian (Diagrammatic), a b c Labial 

 cartilages. I Mandibular arch, o Upper, u Lower 

 portion. II llyoid arch. Ill — VIII Branchial 

 arches. 



mg 



were 

 some 

 deal- 

 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 



