582 The Animal Kingdom 



mously elongate, some resemble round balls, and some look like slender 

 ribbons attached to huge gaping jaws. In habits, the fish are equally 

 varied. Some care for the young to the extent of building complex 

 nests or even to retaining the eggs within the mouth until hatching; 

 others, as a contrast, simply strew their eggs at the water's surface and 

 rely for their survival on the fact that enormous numbers of eggs are 

 produced. While many fish live rather solitary lives, coming together 

 only for mating or feeding, others travel in large groups or schools, and 

 some have developed complex commensal relationships with other ani- 

 mals. Among these last forms are some which dwell in the spongocoel 

 of the sponges. 



Characteristics of the Glass Osteichthyes. — Nearly all the fish 

 belonging to this class possess ossified skeletons with the notochord re- 

 duced to pads between the well-developed vertebrae. There are both 

 median and paired fins which are supported by fin rays. The terminal 

 mouth has well-developed jaws articulated to the skull. Usually there 

 are numerous teeth present. The skin surface is covered with scales of 

 the cycloid or ctenoid type in most species, and there are many mucous 

 glands. The gills of each side are in a common chamber covered with 

 a bony operculum. The paired olfactory organs usually do not open 

 into the mouth cavity, and the eyes are without lids. Many species 

 possess a hydrostatic organ, the simm bladder, which in some few forms 

 serves as an accessory respiratory organ. The heart has three cham- 

 bers, with a single auricle, ventricle, and sinus venosus, and carries 

 only venous blood. The sexes are always separate, and the gonads are 

 paired. Species are either oviparous or ovoviviparous. Some possess 

 larvae quite unlike the adults, but most have direct development. 



The Subclasses of the Class Osteichthyes. — Two large divisions 

 are recognized among the members of this varied class. 



Subclass I. Actinopterygii. Without internal nostrils, with paired fins usu- 

 ally supplied with rays. 



Superorder 1. Chondrostei. The so-called ancient fish, with at present 

 only a few living representatives. The internal skeletal supports are 

 fewer in number than the fin rays of the dorsal and anal fins. Ex- 

 amples : Sturgeon, Acipenser; spoonbill, Polyodon; bichir, Polypterus. 



Superorder 2. Holostei. With modified heterocercal tail, shorter jaws. 

 Examples : bowfin, Amia; and gar, Lepisost/eus. 



Superorder 3. Teleostei. Modern bony fish which possess homocercal 

 tail. Example : perch, Perca. 



