432 FISHES CHAP. 
there is a hyoidean hemibranch supplied with venous blood from 
the ventral aorta. The. gill-filaments are attached throughout 
their length to the interbranchial septa. There is an optic 
chiasma. An air-bladder is not developed. The intestine has a 
spiral valve, and there is a cloaca. The gonoducts in both sexes 
are derived from the kidney system. The ova are large, few in 
number, and enclosed in horny egg-cases, and they are fertilised 
before extrusion. The segmentation is meroblastic, and the 
embryo is furnished with long external gills. 
The Elasmobranchs are for the most part active predaceous 
Fishes, living at different depths in the sea, from the surface to 
nearly a thousand fathoms, and ranging from mid-ocean to the 
shallower waters round the coasts in almost every part of the 
world. Although typically marine, they sometimes ascend rivers 
beyond the reach of tides, and a few are permanent inhabitants 
of fresh water. They are most abundant in tropical and sub- 
tropical areas, where they also attain their greatest size, and are 
numerous in temperate regions, but there are some species which 
are typically Arctic. None of them are small, and some of the 
Sharks are the largest of living Fishes. All are carnivorous, but 
so diversified is their food that in different species it may range 
from other Fishes of no mean size to Molluscs, Crustaceans and 
other Invertebrates, or even to plankton. In their breeding habits 
the Sharks and Dog-Fishes present many interesting features. 
Unlike the generality of Fishes, the eggs are fertilised internally 
as a sequel to the copulation of the sexes. For this purpose the 
males are furnished with special intromittent organs, the myxo- 
pterygia or so-called claspers, which are developed as modifica- 
tions of the hinder portions of the pelvic fins! Each clasper is 
supported by an internal skeleton, consisting of several cartilages 
derived from the radialia of the fins, and is traversed along its 
inner aspect by a groove. When sexual congress takes place the 
claspers are thrust through the cloaca of the female into the ovi- 
ducal orifices, and in some instances it is probable that they are 
retained in this position by hook-like denticles developed at 
their free extremities. The seminal fluid then flows along these 
conduits into the oviducts, in the upper portions of which it meets 
and impregnates the eggs. After fertilisation the egg is enclosed 
in a dark brown horny egg-case, secreted by the oviducal gland. 
1 Den Danske Ingolf-Expedition, ii. No. 2, Copenhagen, 1898. 
