84 COMPARATIVE ANATOMY 



In the orbito-temporal region, a basisphenoid, a prcsphenoid, 

 alisphenoids, and orbitosplienoid s may occur. In the ethmoidal 

 region lateral ^cthmoids and pre-ethmoids are present in bony Fishes 

 and a single ethmoid in Mammals. 1 



The quadrate region of the palatoquadrate usually becomes 

 ossified as a quadrate bone. In Bony Ganoids and Teleosts there 

 is also a metapterygoid, and an autopalatine at the anterior end of 

 the palatine bar which mostly fuses with a dermopalatine. 



An articular is usually formed at the proximal end of Meckel's 

 cartilage, and anteriorly and posteriorly to this zone of ossification 

 an autocoronary and other bones may be developed (Teleosts). 

 The anterior end of Meckel's cartilage usually becomes ossified as 

 a mentomandibular (mentomeckelian) which may become fused with 

 the dentary. 



In the hyobranchial skeleton, the individual segments may 

 be uniformly ossified (e.g. styloliyal, glossohyal, and the segments of 

 the branchial arches) ; but frequently several ossifications may occur 

 in a single segment. 



SPECIAL PART. 



In Amphioxus the vestigial brain is merely surrounded by a 

 thin layer of connective tissue, and there is no proper cranial 

 skeleton. The margin of the oral funnel and the cirri arising from 

 it are supported by cartilage-like skeletal rods. The branchial 

 skeleton consists of a series of elastic rods of a cuticular nature, 

 which are connected together dorsally by arched portions and by 

 transverse bars at different levels, but which remain separate 

 ventrally. A comparison between these and the branchial skeleton 

 of higher forms is rendered all the more impossible by the fact 

 that no definite boundary between the head and trunk can be 

 recognised. 



Fishes (including Cyclostomes). 



The skull of Fishes exhibits very great differences in the various 

 groups, and in many cases reaches a high degree of complication. 

 It is therefore only possible to give here the merest sketch of its 

 characteristic structure in the different Orders. 



In Cyclostomes, the skull is developed essentially in the 

 manner already described. Later, however, it shows many special 

 peculiarities (Fig. 64), probably in consequence of the suctorial 

 (Petromyzon) or parasitic (Myxine) mode of life of these animals : 

 the most important of these is the absence of jaws such as are 



1 It is still uncertain as to how far the replacing bones of the cranium called 

 by the same names in different Vertebrate groups are really homologous. 



