184 EVOLUTION OF ELASMOBRANCHS vi. 6- 



latcral plate muscle, innervated by cranial nerves. Several species of 

 Raja have weak electric organs perhaps used for guidauce (p. 253). 



Life on the bottom has produced many further modifications in the 

 skates and rays. In those that live in shallow and hence well-illuminated 

 waters the colour of the upper surface is often elaborate, the under 

 side being white. In certain species of Raja, for example, there is a 

 pattern of black and white marks, which probably serves to break up 

 the outline of the fish. 



The eyes of the skates and rays have moved on to the upper surface 

 of the head and are protected by well-developed lids. In most forms 

 the pupil is able to vary widely in diameter and often has an operculum 

 by which the aperture can be reduced to two small slits. 



There is a special modification of the respiratory system so that 

 water is drawn in not through the mouth but by the spiracle, which is 

 provided with a special valve that shuts at expiration, as the water is 

 forced out over the gills. The Hypotremata have therefore developed 

 many special features for their bottom-living habits and have diverged 

 among themselves into many varied lines. They have been very 

 successful and are among the commonest fishes in the sea. 



7. Chimaera and the bradyodonts 



Finally we must consider an aberrant group, the bradyodonts, which 

 diverged from the main stock at least as early as the Carboniferous 

 and preserves for us today some features of elasmobranch life at that 

 time as the strange Chimaera, the rat-fish of deep seas (Fig. 115). 

 Instead of the usual large, toothed mouth these Holocephali have a 

 small aperture surrounded by lips, giving the head a parrot-like 

 appearance. The teeth are large plates firmly attached to jaws, and 

 the upper jaw is remarkable in being fused to the skull ('holostylic'), 

 the hyoid arch being free. There is no stomach or spiral intestine. 

 These peculiarities are probably associated with a capacity to eat 

 small pieces of animal food. The Holocephali differ further from the 

 Selachii in the presence of an opercular flap attached to the hyoid 

 arch. There are also extra claspers in front of the usual pelvic ones 

 and an organ on the head of the male known as the cephalic clasper, 

 whose function is obscure. The notochord is unconstricted and the 

 vertebrae reduced to separate nodules. The cleavage is holoblastic, 

 as in other fishes with features of mesozoic type (p. 180). 



Many of the internal features resemble those of selachians, for 

 instance the conus arteriosus, and urinogenitals in which there are 

 separate urinary and spermatic ducts. The brain has a peculiar shape 



