A HIiTORY OF VERTeBkAteS: FISHES 



435 



placoid scale and a tooth is very close. Placoderms, in contrast to the 

 Chondrichthyes, had no teeth, or only a few on the lower jaw. The 

 dorsal part of the hyoid arch, the hyomandibuiar, extends as a prop 

 from the otic capsule of the cranium to the angle of the jaw, and the 

 ventral part of the arch continues into the floor of the mouth. The 

 postmandibular gill slit, which was complete in placoderms, is reduced 

 to a dorsal spiracle or is lost. Five branchial arches lie behind the 

 hyoid in most species, and five typical gill slits open independently to 

 the surface. 



The visceral organs of the dogfish, Squalus acanthias (Fig. 22.8), 

 are in many ways more characteristic of primitive fishes than are those 

 of the specialized cyclostomes. The mouth cavity is continuous poste- 

 riorly with the pharynx. A spiracle, containing a vestigial gill, and the 

 gill slits, containing functional gills, open from the pharynx to the 

 body surface. A wide esophagus leads from the back of the pharynx 

 to a J-shaped stomach. A short, straight valvular intestine continues 

 back to the cloaca. The valvular intestine receives secretions from the 

 liver and pancreas, and contains an elaborate spiral fold known as the 

 spiral valve. This helical fold serves both to slow the passage of food 

 and to increase the digestive and absorptive surface of the intestine. 



The heart consists of a series of chambers arranged in linear se- 

 quence. Blood from the veins, low in oxygen content, enters the pos- 

 terior end of the heart and is pumped out the anterior end into an 

 artery that leads to capillary beds in the gills. Aerated blood from the 

 gills is collected by a dorsal aorta and carried to the body wall and 

 visceral organs. Such a circulatory system is a sluggish, low-pressure 

 system, for the pressure built up by the beating of the heart is im- 

 mediately reduced by friction in the gill capillaries. 



The kidneys are elongate organs drained, as in the frog, by Wolffian 

 ducts. They play a major role in water balance and excretion; however, 

 the gills help eliminate much of the nitrogenous waste. The Wolffian 

 ducts also carry sperm in males. 



At the time of reproduction, eggs are discharged from the ovary, 

 pass through a part of the coelom, and enter the oviducts. The male 

 cartilaginous fish uses a clasper to deposit sperm in the oviducts and 

 fertilization is internal. A horny protective capsule is secreted around 

 the fertilized eggs by certain oviducal cells, and, in all of the skates, the 



Wolffian duct 

 Rectal gland- 





Gill slits 

 ^Si^sx..,. r— Spira-cle 



Mou.lh 



'Cloaca. "-Pancr-e-as Heart ^Pericardial cavity 



Figure 22.8. The visceral organs of the dogfish. 



