Xllt 



PHYLUM CHORDATA 



193 



the uUier on the anterior face of the fourth branchial arch. The 

 fifth branchial arch is, as usual, gill-less, and there is no cleft 

 between it and its predecessor. The gill-filaments are fixed in 

 their whole length to an interbranchial septum, as in Elasmo- 

 branchs. 



The small heart resembles that of the Dog-lish in all essential 

 respects, being formed of sinus venosus, auricle, ventricle, and 

 conus arteriosus, the last with three rows of valves. 



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sue I -, oc.cn 

 rtcn- 



:. 8-00. — Callorhynchus antarcticus, sagittal section of skull ; the labial cartilages are 

 removed. «.. ■-■•. «•. apertures through which the anterior semicircular canal passes from the 

 cranial cavitj- into the auditory capsule ; c. I. d. aperture for endolymphatic duct ; mck-. c. 

 jMcckel's cartilage ; )and. t. mandibular tooth ; ncli. notochord ; Nv. J, trigeminal foramen ; 

 Kr. .'i. 0. foramen for exit of ophthalmic nerves , J\'c. .7.'o', canal for ophthalmic nerves with 

 apertures of entrance and exit ; Nv. 10, vagus foramen ; oc. en. occipital condyle ; or. 

 fenestra separating cranial cavity from orbit ; pat. qv. palatoipiadrate ; pal. t. palatine tooth ; 

 'pn. position of pineal body ; pi. pit for extra-cranial portion of pituitary body ; p. s. r. apertures 

 through which the posterior semicircular canal passes into the auditory capsule ; qu. quadrate 

 region of palatoquadrate ; r. rostrum ; mc. depression for sacculus ; s. t. sella tiu'cica ; tr. 

 tritor ; ro. t. vomerine teeth. 



The brain (Fig. 851), on the other hand, is very unlike that of 

 Scyllium, but presents a fairly close resemblance to that of 

 Scymnus. The medulla oblongata {med. ohl.) is produced laterally 

 into large frill-like restiforui hodies (cp- ''■'^^■)y which bound the hinder 

 half of the cerebellum {d>lm). The diencephalon (^fe/t.) is extremely 

 long, trough-shaped, and very thin-walled, without pronounced 

 optic thalami ; it is continued without change of diameter into a 

 distinct prosencephalon, which gives off the cerebral hemi- 

 spheres {(-rh. h.) right and left. The combined diacoele and proso- 

 cuele {di. cce.) are widely open above in a brain from which the 



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