178 ZOOLOGY sect. 



The Sharks have a prominent tongue supported by the median basi- 

 hyal ; this is entirely or almost entirely absent in the Rays. 

 The various divisions of the enteric canal are similar in 

 all the members of the class to what has already been 

 described in the case of the Dog-fish. A spiral valve is always present 

 in the large intestine, though its arrangement varies considerably in 

 the different families. In some cases (e.g. Carcharias), the fold is 

 not a spiral one, but, attached by one edge in a nearly longitudinal 

 line to the intestinal wall, is rolled up in the shape of a scroll. 

 A caecum occurs in Lccmargus. The rectum always terminates in 

 a cloaca, into which the urinary and genital 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 

 representative 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 Notidanidse a gill on the mandibular 

 side of the spiracular cleft — the spiractdar gill — represented in 

 many others by a retc viirabile or network of blood-vessels. In 

 Selachc (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 hectrt 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. 90), 

 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 depression of greater or less depth, indicating a 

 division into two lateral portions. In Scyllium, as already pointed 

 out, there is a median prosocoele which gives rise anteriorly to 

 two lateral ventricles, or paracoeles, and the same holds good 

 of Rhhia and Acanthias. In most Rays there is only a very small 

 prosoccsle without anterior prolongations; in Myliobatis this is 

 absent. The olfactory bulbs are of great size, in some cases with 



