CH. XX] MAIN DIVISIONS 453 



a fish; TTTcpvyiov, a little wing), the other type of limb as the 

 cheiropterygium (x et 'p> the hand). Pisces therefore are defined 

 by the possession of ichthyopterygia. 



The median and the paired fins are stretched on a skeleton with 

 a two-fold origin, (i) a median series of cartilaginous or bony rods or 

 pterygiophores which support the basal part of the fins, and 

 (ii) a double series of horny fibres or bony dermal fin-rays which 

 support the distal part of the fins. The horny fibres are termed 

 ceratotrichia, the bony dermal rays lepidotrichia; both are 

 from the modification of the superficial layer of the dermis which 

 immediately underlies each side of the fin-blade. 



Taking a broad view of the various types of fish living at the 

 present time, we find that the old common-sense division of the 

 group into Cartilaginous fish or Chondrichthyes and Bony Fish 

 or Osteichthyes is endorsed by the latest scientific writers on 

 the subject. Cartilaginous fish have no true bone ; calcareous 

 matter may be, and often is deposited in the cartilage which is then 

 said to be calcified, but the characteristic structure of cartilage 

 persists. They are further characterised by possessing a peculiar 

 form of scale termed the placoid which is a hollow .tooth-like 

 structure. They never possess an outgrowth from the alimentary 

 canal containing air termed an air-bladder, the nostril or opening 

 to the nasal sac is undivided and the eggs are few and large and 

 are fertilised internally and have already undergone a considerable 

 portion of their development when laid. In many, perhaps in most, 

 cases the egg shell is absorbed in the oviduct and the embryo 

 derives nourishment from the secretions of the oviduct and grows to 

 a relatively enormous size before being bom. To give an instance : 

 the Canadian Spiny Dog-fish Squalus acanthias attains a length of 

 three or four feet and its ripe embryo may be eighteen inches in 

 length. 



In Osteichthyes on the contrary the bone is always present and 

 the cartilage of the skull is always replaced by it to a greater or 

 less extent : an air bladder is developed as an outgrowth from the 

 alimentary canal behind the gill region, the opening to the nasal 

 sac is divided into two by a bridge of bone : placoid scales may be 

 present and indeed usually are present in the region of the stomo- 

 daeum but they are underlain and connected together by calcifications 

 of the dermis which constitute the real effective scales of the 

 Osteichthyes. The eggs are always small and in the vast majority 

 of cases are fertilised externally and pass through a long larval 



