THE GENERAL CHARACTERS OF FISHES. 411 



some extinct forms the fin has a central segmented axis, 

 from which, in Ceratodus for instance, two series of lateral 

 fin-rays arise. This type is called an archipterygium. In 

 Elasmobranchs and other fishes the fin rays radiate outwards 

 from several basal pieces, and there is no median axis. This 

 type is called an icthyopterygium. It is not certain which 

 type is the more primitive. 



The skin almost always bears many scales, in great part 

 due to the dermis. They vary greatly in form and texture. 

 The skin also bears lateral sense-organs. Its glandular 

 cells are not compacted into glands. The dermis is not 

 muscular. In eels and electric fishes the scales are sup- 

 pressed, and in some other cases they are rudimentary. 



The heart is two-chambered, consisting of an auricle 

 which receives impure blood from the body, and a ventricle 

 which drives it by a ventral aorta to the gills, whence the 

 conditions of blood-pressure cause the purified blood to flow 

 to the head, and by a dorsal aorta to the body. In addition 

 to the two essential chambers of the heart, there is a sinus 

 venosus which serves as a porch to the auricle, and there 

 often is a muscular conus arteriosus in front of the ventricle, 

 or a bulbus arteriosus at the base of the ventral aorta. 

 There is no vein which exactly corresponds to what is known 

 from Amphibians onwards as the inferior vena cava. In 

 Dipnoi the auricle of the heart is divided into two, and the 

 circulation is in some respects peculiar. 



There is no distinct indication of an outgrowth from the 

 hind end of the gut comparable to that which forms the 

 bladder of Amphibians, and the allantois of higher Verte- 

 brates. 



Most Fishes lay eggs which are fertilised in the water. 



Life of Fishes. — A fish swims by lateral strokes of its tail 

 and body, as a boat is propelled by an oar from the stern. 

 The paired fins help it in ascending or descending, in steer- 

 ing and balancing. In a few cases, as in the climbing perch 

 and Periophthalmus, the fore-fins are used for scrambling. 



The characteristic form of the body, as seen in herring or 

 trout, is an elongated laterally compressed spindle, thinning 

 off behind like a wedge. It is evident that this form is well 

 adapted for rapid progression through the water. Flat- 

 fishes, whether flattened from above downwards like the 



