XIII 



PHYLUM CHORDATA 



241 



Teleostomi, with a single slit-like branchial aperture behind it. 

 There are no spiracles. There is a well-marked lateral line. 



Endoskeleton. The spinal column (Fig. 915) is represented by 

 a persistent notochord, enclosed in a thick fibrous sheath, together 

 with neural and haemal arches. 



A series of neural or basidorsal cartilages form the bases of the 

 neural arches, and haemal or basiventral cartilages are similarly 

 related below to a series of pleural ribs in the precaudal region, and 

 to a series of haemal arches in the caudal. These two sets of basal 

 cartilages are not precisely opposite throughout, and regularly 

 alternate for some distance in front. They are embedded in the 

 sheath of the notochord, but no centra are formed, and the noto- 

 chord, though pressed upon above and below by the series of basal 

 cartilages, is not constricted in the usual annular manner. At the 

 posterior end it becomes surrounded by cartilage. The neural and 



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B I 



fupra.se I 



J 7 



<2ent 



m nit. 



FIG. 915. Ceratodus forsteri. Lateral view of the anterior portion of the skeleton. 

 A, anterior median investing bone of the roof of the skull ; B, posterior median investing 

 bone ; C, inner lateral investing bone ; has. basal cartilage of the pectoral fin ; br. branchial 

 arches ; dent, tooth of lower jaw ; hy. hyoid arch ; int. interoperciilum ; lam. plate over- 

 hanging branchial region ; mck. Meckel's cartilage ; occ. rb. occipital rib ; op. operculum ; 

 pal. palatoquadrate ; pet. pectoral arch ; rbs. ribs ; sub. orb. sub-orbital bones ; sq. so-called 

 squamosal ; supra, sc. post-temporal. 



haemal arches are ossified ; each is surmounted by a rod-like neural 

 or haemal spine which forms part of a continuous three- jointed 

 ossified rod, the proximal segment being the spine, and the two 

 others radials. The pleural ribs are curved bony rods extending 

 downwards and somewhat backwards in the body- wall immediately 

 outside the peritoneal membrane, like the pleural ribs of the Teleo- 

 stomes. The first pair the occipital ribs (Fig. 915, occ. rb.), 

 thicker and straighter than the rest, are connected with the skull in 

 its vertebral portion. 



The skull (Figs. 915, 916, and 917) consists of an undivided mass 

 of cartilage, devoid of fontanelles, narrowest between the orbits, and 

 broadening before and behind ; posteriorly it is prolonged into a 

 plate (lam.} overhanging the branchial region. Embedded in the 



