THE SKULL 



73 



or parasitic (Myxine) mode of life of these animals : the most 

 important of these is the absence of jatus such as are present in all 

 other Craniata; for this reason these forms are spoken of as 

 Gyclostomata to distinguish them from the other craniate Verte- 

 brates or G-nathostomata. Instead of the jaw-apparatus, which 

 has doubtless become degenerated, and indications of which as well 

 as of the hyoid can apparently still be seen (Fig. 54, sb.oc.a, 

 p. lat.c, sty.c, cn.c), a number of cartilages are present supporting 

 the anterior part of the head. In the adult Lamprey, for instance, 

 the suctorial mouth is supported by various skeletal elements, 

 amongst which may be mentioned a ring-like cartilage around the 

 margin of the dome-shaped oral funnel, between the dorsal side 



brb 1 Its 



335in 



l.ci- 



br.cl.1 



^ncli 



Fig. 54. — Skull with Branchial Basket of Petromyzon marinns. 

 (After W. K. Parker.) 



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

 anterior lateral cartilage; aii.r. annular cartilage; aji.r. auditory capsule; 

 br.b. 1 — ~, vertical bars of branchial basket; br.d. 1 — 7, external branchial 

 clefts; cn.c cornual cartilage; cr.r. cranial roof; he. 1 — 4. longitudinal 

 bars of branchial basket; Ig.c. lingual cartilage; m.r.c. median ventral 

 cartilage ; na.ap. nasal aperture ; nch. notochord ; X'o. :i, foramen for optic 

 nerve ; olf. c. olfactory capsule ; pc.c. pericardial cartilage ; p.d.c. posterior 

 dorsal cartilage; p.lat.c. posterior lateral cartilage; sb.oc.a. sub-ocular 

 arch ; nf.p. styloid process ; sty.c. styliforni cartilage ; t. teeth. 



of which and the brain-case are a couple of large overlapping 

 cartilages : the tongue is supported by a long, lingual cartilage. 

 On the mucous membrane covering the annular and lingual 

 cartilages inside the oral funnel are a number of horny teeth. 

 The fibro-cartilaginous olfactory sac is tin]jaircd, and opens 

 on the dorsal surface of the head by a single nostril. The 

 visceral skeleton also shows many exceptional peculiarities : it 

 consists of a delicate cartilaginous basket-work (Fig. 54), and has 

 a very superficial position ; we may .accordingly speak of the 

 unsegmented cartilages of which it is composed as " extra- 

 hranchials" to distinguish them from the true bi'anchial arches of 

 the Gnathostomata. 



