230 



PISCES 



bone, cored with dentin or ivory and tipped with enamel from the 

 epidermis. The enamel is inorganic, the cells being replaced by 

 lime salts. The dentin contains 34 per cent organic matter and the 

 bone is cellular tissue. 



tn.sp 



Fig. 108. Chimaeroid fish. After R. Lull. (Courtesy of The Macmillan Co.) 



There are senso?~y tubes or mucus canals on the ventral surface 

 just under the skin. They function for touch and chemical sense, 

 having ampullae with sensory cells at the inner ends and pores 

 opening to the outside. There are no spines on the ventral surface 

 except a few bristly ones in the region of the cloaca. 



Muscular System. — The muscles are segmentally arranged, the 

 jaw muscles being well developed. Organs not present in the skate 

 are the electric organs, best developed in the teleosts, a South Ameri- 

 can eel {Gymnotus) and an African Siluroid {Malapterurus) , but 

 also found in the Elasmobranch torpedo ray. In the torpedo they 

 lie on each side of the head between the gills and the anterior part 

 of the pectoral fin. They are vertical prisms, divided by transverse 

 partitions of connective tissue into large number of cells formed 

 from metamorphosed muscle fibers. These electropaxes or electric 

 plates consist of muscle substances and many nerve endings. Four 

 nerves connect them with the electric lobe in the medulla oblongata. 



Skeleton. — The skeleton is cartilaginous with a deposition of 

 bone in the jaws and vertebral column. The skull of the skate is 

 not ossified: It consists of a large cartilaginous case with brain 

 cavity; 2 condyles, 1 large ear capsules, 1 large nasal capsules, a 

 long rostrum in front and two fontanelles on the roof. The pectoral 

 girdle is a hoop of cartilage attached dorsally to the crest of the 

 vertebral plate. The ventral region, the coracoid is separated from 



