PISCES. 



VERTEBEATE animals . adapted for an aquatic life, having their 

 extremities modified into fins ; respiring, almost invariably, solely 

 by means of gills * ; possessing a heart with only two (in one 

 instance three) cavities, and being cold-blooded. They are scale- 

 less, or partially or wholly scaled, the scales being sometimes in 

 the form of osseous plates. 



Synopsis of Subclasses occurring in India. 



Skeleton cartilaginous. Skull without cranial sutures. Gills at- 

 tached by their outer edges to the skin, with a gill-opening 

 intervening between every two gill-laminse. Conus arte- 

 riosus contractile. Optic nerves not or only partially de- 

 cussating. A spiral valve to intestines. 



I. CHONDROPTERYGII, 



Skeleton osseous. Skull with cranial sutures. Vertebra? com- 

 pletely separated, and the posterior extremity of the vertebral 

 column bony, or having bony plates. Branchiae free, and the 

 water discharged through a single aperture on each side, 

 protected by a bony gill-cover or opercle ; branchiostegal rays 

 present. A non-contractile bulbus arteriosus, having a pair 

 of valves at its commencement. Optic nerves decussating. 



H. TELEOSTEI. 



Skeleton membrano- cartilaginous and notochordal. Skull and 

 brain absent. Pulsating sinuses present in place of a heart. 



III. LEPTOCARDII. 



* Certain fish, as the Ldbyrinthici and Ophiocephalida, provided they are 

 able to obtain direct access to atmospheric air, can live in water even with a 

 bandage fastened round their gill-covers, entirely preventing their gills being 

 employed for respiratory purposes. Such a proceeding would, however, be 

 fatal to the majority of fishes, thus showing that some forms possess means of 

 depurating their blood which are n ot present in all. 



B 



