324 THE DOG-FISH 



Still another characteristic feature is the presence, in 

 addition to the dermal exoskeleton, of an endoskeleton^ or 

 system of internal supporting structures. Between the 

 cerebro-spinal cavity above and the coelome below is a 

 longitudinal series of biconcave discs or vertebral centra 

 ( V. Cent) : they are formed of a peculiar tissue called 

 cartilage or gristle, and are strongly impregnated with lime- 

 salts : in the young condition their place is occupied by a 

 gelatinous rod, the notochord. The centra, which alternate 

 with the muscle-segments, are connected with a series of 

 cartilaginous arches (n.a) which extend over the cerebro- ' 

 spinal cavity and with the centra constitute the vertebral 

 column. In the tail there is also a ventral series of arches 

 (h.a.) enclosing a space (H.C] which indicates a backward 

 extension of the ccelome in the embryo. 



Anteriorly the vertebral column is continued into a 

 cartilaginous box, the cranium (Cr) which encloses the brain 

 and the organs of smell and hearing. The jaws, referred to 

 above, are cartilaginous rods which bound the mouth above 

 and below. The gills are supported by a complicated 

 system of cartilages (Br. A, Br. R, Br. JR.') and both 

 median and paired fins by parallel rods of the same 

 material (Ptgph). All these cartilages are strengthened 

 by a more or less extensive superficial deposit of bony 

 matter. 



The mouth (MtJi) leads into a large oral cavity (Or. cav) 

 which passes insensibly into a wide throat or pharynx (P/i) : 

 from this a short gullet (Gut) leads into a large U-shaped 

 stomach (St\ whence is continued a short wide intestine 

 (Int) opening on to the exterior through the intermediation 

 of a small chamber, the cloaca (6V). From the gullet 

 backwards the enteric canal is contained in the coelome. 

 The greater part of the enteric epithelium is endodermal : 



