xm PHYLUM CHORDATA 179 



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

 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. 



The brain (Fig. 803), on the other hand, is very unlike that of 

 Scyllium, but presents a fairly close resemblance to that of 

 Scymnus. The medulla oblongata (ined. 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, having no indication of 

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

 distinct prosencephalon, which gives off the cerebral hemi- 

 spheres (crb.h.) right and left. The combined di- and proso- 

 coeles (di. cce.) are widely open above in a brain from which the 

 membranes have been removed (A), but in the entire organ (B) 

 are roofed over by a conical, tent-like choroid plexus (ch.plx. 1). 

 The cavities of the small, spindle-shaped hemispheres (crb. Ti.) com- 

 municate with the prosoc'oele by wide foramina of Monro (for. M.). 

 partly blocked up by hemispherical corpora striata (cp. sir.). Each 

 hemisphere is continued in front into a delicate thin- walled tube, 

 the olfactory peduncle (olf.p.), bearing at its extremity a com- 

 pressed olfactory lobe (olf. /.). 



The optic nerves (Nv. 2) form a chiasma. The pineal body(pn. b.) 

 is a small rounded vesicle borne on a hollow stalk {pin,, s.) which 

 runs just outside the posterior wall of the tent-like choroid plexus. 

 The pituitary body (pty.) consists of intra- and extra-cranial por- 

 tions, the former lodged in the sella turcica, the latter in the pit, 

 already noticed, on the ventral or external face of the skull-floor 

 (Fig. 802, pt.). In advanced embryos the two are united by a 

 delicate strand of tissue. 



Urino-genital Organs.- -The kidneys (Fig. 804, M.) are lobed, 

 deep-red bodies, like those of the Dogfish, but shorter and stouter. 

 The female organs, also, are constructed on the Elasmobranch 

 pattern, and are chiefly noticeable for the immense size of the 

 shell-glands and of the uteri. But the male organs present 

 certain quite unique characters. The testes (ts.) are large ovoid 

 bodies the tubules of which do not contain fully developed sperms, 

 but only immature sperm-cells. These latter are passed through 

 the vasa efferentia into the immense epididymes (epid.), where they 

 become aggregated into spermatophores in the form of small ovoidal 

 capsules surrounded by a resistent membrane and full of a gelatinous 

 substance in which bundles of sperms are imbedded. The lower 



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