10 



VERTEBRATA 



PHYLUM VIII 



Iii all cycloid and ctenoid fishes the caudal fin is externally and internally 

 homocercal (Fig. 21). The vertebral column terminates in front of the fin 

 with a vertebra bearing a broad, vertical, fan-shaped plate (hypural), which is 

 formed by the fusion of several fin supports and haemapophyses. This 

 terminal plate generally encloses a short, upwardly directed remnant of the 

 notochord or a bony style (urostyle). There is thus an internal heterocercy 

 as the basis of the homocercal tail. 



Nearly all Palaeozoic fishes have a diphycercal or heterocercal tail ■ hemi- 

 heterocercal forms begin with Acentrophorus in the Upper Permian, and are 

 common in the Triassic and Jurassic. The Cretaceous, Tertiary, and Recent 

 bony fishes, when adult, exhibit a homocercal tail ; but in their immature 

 condition there is always a diphycercal or heterocercal stage. 



Skull, — The head forms the anterior end of the vertebral axis, and its skeleton 

 exhibits a greater complexity in fishes than in the higher Vertebrates. Except 



Kr Po 



Fig. 22. 



Skull and branchial apparatus of Squatina vulgaris, Risso. Recent. BR, Rays of the hyoid arch ; CBr, Cerato- 

 branchial ; Co, Copula of the hyoid arch ; Co', Hindermost copula of the branchial arches ; EBr, Epibranchial ; 

 Etli, Ethmoidal region ; Hi/, Hyoid arch ; HBr, Hyobranchial ; HyM, Hyomandibular ; L, U, L", Anterior (pre- 

 niaxillary), posterior (maxillary), and inferior (preniandibular) labial cartilages ; Md, Lower jaw or mandible ; Occ, 

 Occipital region ; Po, Postorbital process ; PQ, Palatoquadrate. III-VII, Branchial arches (after Gegenbaur). 



in Amphioxus, the brain is enclosed in a cartilaginous or bony capsule, the skull 

 or cranium, which also contains the organs of hearing, sight, and smell. With 

 the skull are connected a large number of paired cartilaginous or bony pieces, 

 which together form the so-called visceral skeleton. 



The embryonic primordial cranium of all Vertebrates is cartilaginous, and 

 is developed like the vertebral column round the notochord, which always 

 extends far into the base of the skull. 



Among the Selachii (Fig. 22) the whole cranial skeleton is cartilaginous, 

 and only exceptionally hardened by the deposition of phosphate of lime 

 (Ichthyotomi). The long cranial capsule does not exhibit any sutures, and 

 comprises in its anterior part (ethmoidal region) the olfactory nerves, in the 

 middle (orbital region) the optic nerves and eyes, in the hinder part the 

 organs of hearing and the hemispheres of the cerebrum and cerebellum. The 

 visceral skeleton consists of a number of variously shaped cartilaginous arches, 

 of which the foremost (palatoquadrate) surrounds the cleft of the mouth, and 

 is characterised by its relatively large size. The two movable branches of 



