60 CLASS PISCES. 



the membrana elastica externa and unite with each other, and 

 so reinforce the vertebral centra. When the chordal sheath is 

 inconspicuous, and the centra appear to be mainly derived 

 from the arch tissue, the vertebral column is said to be arci- 

 centrous {Ganoidei, Teleostei).* 



In this way a segmented vertebral column is established. In 

 Teleosteans and bony Ganoids a further complication is added 

 in the replacement of the cartilage by osseous tissue. The 

 centra in the trunk (Fig. 34) carry as a rule short transverse 

 processes, which may be called haemal arches, though they do 

 not meet ventrally except in the caudal region, where they en- 

 close a space containing the caudal artery and vein. The ribs 

 are never more than short pieces of cartilage or bone attached 

 to the outer ends of the transverse processes in the trunk 

 region. There is no sternum in fishes. 



The primitive craniumf consists of a cylinder of continuous 

 cartilage, to which are attached anteriorly the nasal capsules 

 widely open below, and posteriorly the auditory capsules. It is 

 thus divided into four regions ; the occipital surrounding the 

 foramen magnum, the wide auditory region, the narrow sphen- 

 oidal or interorbital region, and the wide nasal or ethmoidal region. 

 The junction of the parachordal with the trabecular region of 

 the skull is marked externally by the foramen in the median 

 floor, which transmits the internal carotid arteries (Fig. 36, 13), 

 and internally by the posterior clinoid ridge which forms the 

 hinder wall of the fossa for the lodgment of the pituitary body 

 (Fig. 36, 6). In the embryo two elongated cartilages — the para- 

 chordal cartilages — are developed on each side of the cranial part 

 of the notochord. They unite with each other around the 

 notochord and form the basilar plate which gives rise to the 

 occipital and part of the sphenoid regions. The auditory capsules 

 which are developed round the membranous labyrinth become 

 fused with this part of the skull. The anterior end of the skull 

 in front of the pituitary fossa is formed by a second pair of 

 embryonic cartilages, the trabeculae. To the front end of these 

 the nasal capsules become attached, thus giving rise to the 

 ethmoidal region. 



* Vide Gadow, Phil. Trans., 186, 1895, p. 165. 



t C. Gegenbaur, Untersuchungen z. vergl. Anat. d. Wirbellhiere, 

 Heft 3. Leipzig, 1872. Id., " Ueb. d. Occipitalregion, etc. der Fische." 

 Kolliker'a Festschrift, Leipzig, 1887 



