OF CARTILAGINOUS FISHES. 
«5 
on 
2iark tribe ; and among a third clafs, they are fimated 
Ihe top of the head, as exemplified by the pipe fifh. 
from thefe foramina, there are two bending cylindri- 
cal dudts, that run to the lungs, and arefuppofed to con- 
ve y the air that gives their organs their proper play : 
heart, however, has but one valve ; and hence, their 
klood does not perform that double circulation which ob- 
* ain among the cetaceous kinds, and the lungs feem ra- 
ther an internal affiftant to the gills, than for fupplying 
*^ e fame purpofe which they do in quadrupeds, for they 
' t! ant both the pulmonary vein, and artery. 
Another ftriking charafter by which this order of the 
‘’mural kingdom may be diltinguillied, is, that their fledi 
ts fupported by cartilages inftead of bones. In the ceta- 
c^ous tribes we have feen, that the bones were hard, and 
filled 
with marrow, rcfembling thofe of quadrupeds, 
While thefe parts in the fpinous tribes, are fmall, flender, 
an d pointed, refembling thorns, and generally folid 
throughout. The fiz.e of other animals increafes with 
l beir age, and Hops at a certain point ; but from the pli- 
“ nc y of the bones in this tribe, they feem to have no 
°m>ds placed to their dimenfions, and it is fuppofed 
f bat they grow larger every day till they die f. 
A third general character of thefe liihes arifes from 
tbeu manner of generation ; fome of them are oviparous. 
While the greater part produce living young ; the fame 
U P ’city of character attending them in this refpeit, 
^hich we have already taken notice of with regard to 
eir conformation. In thofe that are Viviparous, the 
? r °geny i s excluded from an egg hatched within the bo- 
dy 
1 Galdftnith’s Mat. Hift. 
vol. it. p. 231. 
X 
