Mil 



PHYLUM CHOKDATA 



187 



passing in a straight line from gullet to anus ; there is a well- 

 developed spiral valve in the intestine. 



Respiratory Organs. There are three pairs of holobranchs or 

 complete gills borne on the first three branchial arches, and two 

 hemibranchs or half -gills, one on the posterior face of the hyoid, 

 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 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. 



or 



, 



oc.cn 



FIG. 861. 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. gn. 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 ; . t. 

 sella turcica ; tr. tritor ; vo. t. vomerine teeth. 



The brain (Fig. 862), 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 



