5 i6 



PISCES— FISHES. 



ol.o. 



The ventricle of the heart has an anterior auxiliary region— 

 a contractile conus arteriosus. The males are provided 

 with copulatory modifications of the hind-limb, known as 

 claspers. Fertilisation is internal. The ova are few and 



large. Large egg-purses 

 are common, but some 

 Elasmobranchs are vivi- 

 parous. The embryos 

 have external gills. 



Subdivisions. — The 



shark and the skate are types 

 of two distinct suborders : (i) 

 The older Selachoidei, with 

 approximately cylindrical 

 bodies and lateral gill-open- 

 ings, as in shark and dog-fish ; 

 (2) the more modified Bato- 

 idei, with flattened bodies, 

 ventral gill - openings, and 

 pectoral fins joined to the 

 head, as in skates or rays. 



Special forms. — Mus- 

 telus, Carcharias, Squalus, 

 Torpedo, Acanthias, and 

 others, are viviparous ; Raja, 

 Scyllium, Cestracion, and 

 others, are oviparous. In 

 two species of the genera first 

 named there is a placenta-like 

 connection between the yolk- 

 sac of the embryo and the 

 uterus of the mother. Zygietia 

 has a peculiar hammer-like 

 head expansion ; Pristis has 

 the snout prolonged in a 

 tooth-bearing saw ; Torpedo 

 has a powerful electric organ. 

 History. — The Elasmo- 

 branchs appear in the Upper 

 Silurian, are very abundant 

 from the Carboniferous on- 

 wards, but are now greatly 

 out-numbered by the Bony Fishes. An increasing calcification of the 

 axial skeleton is traceable through the ages, and in some of the ancient 

 forms the exoskeleton was greatly developed, often including long 

 spines or ichthyodorulites firmly fixed on the dorsal fins or on the neck. 

 Among the most remarkable extinct genera is Pleuracanthus, from 

 Carboniferous to lower Permian. It had a terminal mouth, a naked 



Fig. 219. — Young skate. — From Beard. 



The yolk-sac has been cut off, the yolk-stalk is 

 left, m., Mouth; ol.o., nostril; e.g., exter- 

 nal gills ; a., cloaca ; c, claspers. 



