32 



CYCLOSTOMATA 



forms, and the posterier cartilage to the neural arch (basidorsal) 

 (Fig. 18). In the tail -region the arches are very irregular 

 and small, and finally disappear. In Myxinoids, where there are 

 no such arches, a continuous plate encloses both the notochord and 

 the nerve-chord posteriorly (Fig. 19). The median fin is continuous, 

 or in lampreys an anterior tin becomes separated off'. In all Cyclo- 

 stomes the fin-web is supported by slender median rods of cartilage 

 (Figs. 19 and 28), separate from each other in front, but fusing at 

 their base behind, and then towards the tip of the tail with the 

 plate mentioned above in the Myxinoids. These rods or 'spines' 

 may branch, are several times as numerous as the segments they 



Petromyzon marinus, 1.. Left-side view "f a portion of tin- notochord and neighbouring 

 organs ; the left half has been removed by a median longitudinal section in the anterior region. 

 mi, anterior arch (interdorgal?) ; ao, dorsal aorta ; d.r, dorsal nerve-root ; /, fatty tissue ; /.'.<', 

 kidney vein ; l.n, lateral-line nerve; l.p.c, left posterior cardinal vein; n.c, nerve-chord ; nt, 

 notochord ; p», posterior arch (basidorsal.') ; ?-.</, ramus dorsal is ; r.p.c, right posterior cardinal 1 ; 

 -.". segmental artery ; sh, notochordal sheath : s.v, segmental vein : v.r, ventral nerve-root. 



occupy, and are continued round the end of the tail to the ventral 

 region, where they develop in the same way and support the ventral 

 fin. Since the dorsal rods reach proximalby to the connective-tissue 

 tube surrounding the nerve-cord, and even join together at their 

 bases and (in Myxinoids) with the axial plate, they should probably 

 be considered as forming part of the axial skeleton, as, in fact, pro- 

 longed neural spines. If this view be correct, there is no special 

 appendicular or fin-skeleton in the Cyclostomes (see p. 69). 



In the Petromyzontia the brain -case is partly membranous, 

 especially above, being bridged over dorsally only between the 

 auditory capsules (Fig. 20). These capsules alone are firmly fused 

 to the cranium ; the optic capsules are, of course, free, and the nasal 

 capsules are attached by connective tissue. The floor of the skull 

 is formed by the united parachordals. This plate is continuous 



