20H PHYLUM CHORDATA 173 



ducts also lead. There is always a voluminous liver and a well- 

 developed pancreas. 



A thyroid lies in the middle line behind the lower jaw. A repre- 

 sentative of the thymus lies on either side, a little below the upper 

 angles of the branchial clefts. 



The respiratory organs of the Elasmobranchii always have 

 the general structure and arrangement already described in the 

 case of the Dog-fish. In the Rays the water of respiration is taken 

 in mainly through the spiracles ; in the Sharks through the mouth. 



In addition to the gills supported on the hyoid and branchial 

 arches there is also in the Notidanidee a gill on the anterior side 

 of the spiracular cleft the spiracular gill represented in many 

 others by a rete mirabile or network of blood-vessels (pseudobranch). 

 In Selache (the Basking Shark) there are a series of slender rods, 

 the gill-rakers, which impede the passage outwards through the 

 branchial clefts of the small animals on which those Sharks feed. 



Blood-system. The heart has, in all essential respects, the 

 same structure throughout the group. The conus arteriosus is 

 always contractile, and contains several rows of valves. The 

 general course of the circulation is the same in all (see p. 88), with 

 some variation in the precise arrangement of the vessels. In some 

 of the Rays the ventral aorta and the roots of the afferent vessels 

 are partly enclosed in the cartilage of the basi-branchial plate. 



The brain attains a much higher stage of development than in 

 the Cyclostomata. The fore-brain greatly exceeds the other divisions 

 in size. In Scymnus there are two widely-separated parencephalic 

 lobes or cerebral hemispheres containing large lateral ventricles. 

 In other genera there is at most, as in the Dog-fish, a median depres- 

 sion of greater or less depth, indicating a division into two lateral 

 portions. In Scy Ilium, as already pointed out, there is a median 

 prosoccele which gives rise anteriorly to two lateral ventricles, or 

 paracceles, and the same holds good of Rhina and Acanthias. In 

 most Rays there is only a very small prosoccele without anterior 

 prolongations ; in Myliobatis this is absent. The olfactory bulbs 

 are of great size, in some cases with short and thick, in others longer 

 and narrower, stalks. In Scyllium, Rhina, and Acanthias, as well 

 as in Scymnus, they contain ventricles (rhinocceles) continuous with 

 the paracceles ; in the Rays they are solid. 



The diencephalon is of moderate extent. On its lower aspect 

 are a pair of rounded lobi inferiores, which are of the nature of 

 dilatations of the infundibulum, and a saccus vasculosus, which is 

 a diverticulum of the infundibulum ; directly below the saccus 

 vasculosus lies the hypophysis. The epiphysis is long and narrow. 



In the hind-brain the cerebellum is relatively greatly elongated 

 and overlaps the optic lobes and sometimes also the diencephalon 

 in front, while behind it extends over the anterior part of the 

 medulla oblongata. It usually contains a cerebellar ventricle or 



