BRAIN 41 



mouth (Fig. 24). In the Petromyzontia it enlarges into a sac 

 below the brain (Fig. 34) ; and in the Myxinoidea it actually 

 opens backwards into the pharynx (Fig. 30), passing down 

 between the trabeculae — thus piercing the basal plate from 

 above. 



The brain is very lowly organised (Ahlborn [5], Eetzius [356], 

 Johnston [247]). In Petromyzon the secondary-, fore-, mid-, and 

 hind-brain all have large cavities and non-nervous roofs (Fig. 25), 

 and do not overlap each other. The olfactory lobes are large, and 

 closely applied to the remarkably small cerebral hemispheres, and 

 the cerebellum is rudimentary. The mid-brain, on the contrary, is 

 unusually large. There is but a rudiment of the saccus vasculosus. 

 The epiphysial outgrowths consist of a pigmented pineal eye of 

 elaborate structure in connection with the commissura habenularis, 

 and of a smaller and simpler sac of the same nature below it, the 

 parapineal organ, connected also with the posterior commissure 

 (p. 24). In the larger superior vesicle not only does the outer 

 wall become thickened into a clear cellular lens, but the inner wall 

 develops into a pigmented retina of more perfect structure than 

 that of the smaller inferior parapineal vesicle (Beard [33], Dendy 

 [112], Studnicka [430]). The skull wall thins out above these 

 organs, which are separated from the exterior by a transparent 

 corneal area (Fig. 14). In the Myxinoidea the pineal organs are 

 less developed, and the brain is remarkable for the thickness of its 

 walls, and reduction of the internal cavities (Fig. 141). 



The first gill-cleft, the spiracular slit between the 3rd and 4th 

 somites, is obliterated in the adult Cyclostome (Dohrn [114a], Dean 

 [106]). The remaining branchial slits on each side are seven in 

 number in the Petromyzontia, and from six to fourteen in number 

 among the Myxinoidea. Since, in the Gnathostomes, the pairs of 

 branchial slits rarely reach and never surpass the number seven, 

 the question arises as to whether the Cyclostomes are more 

 primitive in having a larger supply. At present, no definite 

 answer can be given ; but, as the number of slits in Amphioxus 

 is very large, it seems probable that it may have been gradually 

 reduced in the higher forms (p. 95). 



The gills are distinguished by their spherical shape, being sac- 

 like organs, lying to a great extent surrounded by a blood-sinus, 

 and with gill-lamellae set all round the internal wall, scarcely 

 interrupted above and below (Figs. 26, 27). The lining of the gill- 

 sac is derived entirely from an endodermal outgrowth (Goette 

 [169]). The gill-sac communicates externally by a narrow ecto- 

 dermal duct, produced into a tube of considerable length in the 

 Hag-fishes. A narrow internal aperture opens either directly into 

 the pharynx, as in the Myxinoidea (Fig. 30), or into a sub- 

 oesophageal tube in the Petromyzontia (Fig. 34). This branchial 



