SELACHII. 159 



relatively far back, and each of them has a separate external opening. 

 These openings are in the Squalidce on the sides, in the Raiidce on 

 the ventral surface of the body. In the Chimceridce the branchial 

 pouches open on either side into a common gill-slit, over which a 

 cutaneous fold, arising from the suspensorium of the jaw and 

 serving as a branchial operculum, is spread. 



The dentition presents many variations. Sometimes (Hexanchus, 

 Acanthias) the whole of the buccal cavity as far as the entrance to 

 the oesophagus is covered with small teeth of the mucous membrane 

 (placoid scales *) ; sometimes there are larger teeth, which also 

 always belong to the mucous membrane, and are arranged in rows 

 on the rounded edge of the jaw in such a manner that the younger 

 posterior rows of teeth have their points turned inwards, while the 

 teeth of the anterior rows, which are older and more or less worn, 

 have their points turned upwards and outwards. 



In the Squalides, dagger-shaped or saw-shaped serrated teeth pre- 

 ponderate, while conical or flat pavement-like molar teeth are 

 characteristic of the greater number of Raiides. 



Spiracles are frequently present on the upper surface of the head 

 behind the eyes ; they are used for the expulsion of the water from 

 the pharyngeal cavity. The digestive canal is dilated to a spacious 

 stomach, but is relatively short ; the small intestine is furnished with 

 a spirally coined fold of the mucous membrane the so-called spiral 

 valve which considerably increases the extent of the absorbing 

 surface. A swimming bladder is always absent, though the rudiment 

 of it is often discernible. 



The heart * has a nmscular conus arteriosus ; it contains two to 

 five rows of valves, and represents a part of the ventricle which has 

 become independent. 



In the structure of the brain and of the sense organs, the 

 Selachians hold the highest place amongst the fishes (fig. 588). The 

 hemispheres are of relatively considerable size, present longitudinal 

 and transverse impressions, and traces of convolutions on their 

 surface. The cerebellum, also, may be so well developed that the 

 fourth ventricle is almost entirely covered by it. The two optic 

 nerves always form a chiasma and some of their fibres cross. The 

 eyes in the Squalides are not only protected by free lids, but often 

 also by a moveable nictitating membrane. 



* O. Hertwig, Jen. naturn-iss. Zeitschr. Tom. VIII., 1874. 



* C. Gegenbaur, ' Zur vergleichenden Anatomic des Herzens." Jen. natur- 

 iviss. Zeitschr. Tom. II. 



