SKULL 



85 



present in all other Craniata ; for this reason these forms are 

 spoken of as Cydostomata to distinguish them from the other 

 craniate Vertebrates or Gnathostomata. 



In addition to the peculiar histological differences in the struc- 

 ture of the cartilage, many other special characters of the skull are 

 seen not only in the Class as a whole, but also in the two Orders, 

 which must have become specialised very early along different 

 lines, so that a comparison between them is rendered difficult. 

 The low character of the skull is marked by the imperfect develop- 

 ment of the cartilaginous brain-box, and thus most of the cerebral 

 nerves on making their exit from the skull are but slightly or not 

 at all surrounded by cartilage : moreover, the anterior region of 

 the spinal axis, which in Gnathostomes becomes assimilated to the 

 cranium, remains undifferentiated, so that an occipital region is 

 wanting and the vagus makes its exit behind the skull (" palceo- 

 cranium ") and not through its walls. 



The jaw-apparatus has doubtless become degenerated, and 

 indications of its former presence may possibly be recognised. 



sb.oc.a 



I.C.+ 



l.C.3 



br.cl.r 



FIG. 04. SKULL WITH BRANCHIAL BASKET OF Petromyzon marinus. 

 (After W. K. Parker.) 



The cartilaginous parts are dotted. a.d.c, anterior dorsal cartilage; a.lat.c, 

 anterior lateral cartilage; an.c, annular cartilage; au.c, auditory capsule; 

 br.b, 1 7, vertical bars of branchial basket ; br.cl, 1 7, external branchial 

 clefts; cn.c, cornual cartilage; cr.r, cranial roof; l.c, 14, longitudinal 

 bars of branchial basket; Ig.c, lingual cartilage; mv.c, median ventral 

 cartilage ; na.ap, nasal aperture ; nch, notochord ; Nv. 2, foramen for optic 

 nerve; olf.c, olfactory capsule; pc.c, pericardial cartilage ; pclc, posterior 

 dorsal cartilage ; p.Iat.c, posterior lateral, cartilage ; sb.oc.a, subocular arch ; 

 st.p, styloid process ; sty.c, styliform cartilage ; t, teeth. 



Thus certain similarities between the subocular arch (Fig. 64, 

 sb. oc. a.) and the palatoquadrate of the Frog-tadpole were pointed 

 out by Huxley ; the posterior lateral cartilage (p.Iat.c) is perhaps 

 comparable with MeckePs cartilage and the styloid process (st.p) 

 and cornual cartilage (cn.c) with the hyoid. A number of other 

 cartilages supporting the anterior parts of the head cannot well be 

 directly compared with parts of the gnathostomatous skull. 



In the adult Lamprey, for instance, the suctorial mouth is 



