XIII 



PHYLUM CHORDATA 193 



the other 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-fish in all essential 

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

 conus arteriosus, the last with three rows of valves. 



Nv. s.'o' 



rtch ccn 



FIG. 850. Callorhynchus antarcticus, sagittal section of skull ; the labial cartilages are 

 removed, a. s. c. apertures through which the anterior semicircular canal passes from the 

 cranial cavity into the auditory capsule ; e. 1. d. aperture for endolymphatic duct ; mck. c. 

 Meckel's cartilage ; mnd. t. mandibular tooth ; nch. notochord ; Nv. 5, trigeminal foramen ; 

 Nv. 5. o. foramen for exit of ophthalmic nerves , Nv. 5.'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 ; pal. qu. palatoquadrate ; pal. t. palatine tooth ; 

 pn. position of pineal body ; pt. pit for extra-cranial portion of pituitary body ; p. s. c. apertures 

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

 region of palatoquadrate ; r. rostrum ; sac. depression for sacculus ; s. t. sella turcica ; fr 

 tritor ; to. 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. obi.) is produced laterally 

 into large frill-like restiform bodies (cp. rst.\ which bound the hinder 

 half of the cerebellum (cblm.). The diencephalon (dien.) 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 (n'b. li.) right and left. The combined diacoele and proso- 

 ccele (di. civ.) are widely open above in a brain from which the 



VOL. n N 



