CARTILAGINOUS FISHES. 259 



witnout any bone to open and shut them, as in spinous fishes ; from 

 which, by this mark, they may be easily distinguished, though other- 

 wise very much alike in appearance. From these aie bending cylin- 

 drical ducts, that run to the lungs, and are supposed to convey the 

 air, that gives the organs their proper play. The heart, however, has 

 but one valve; so that their blood wants that double circulation which 

 obtains in the cetaceous kinds; and the lungs seem to me rather as 

 an internal assistant to the gills, than fitted for supplying the same 

 offices as in quadrupeds, for they want the pulmonary vein and artery. 



From this structure, however, the animal is enabled to live a longer 

 time out of water than those whose gills are more simple. The car- 

 tilaginous shark, or ray, live some hours after they are taken ; while 

 the spinous herring or mackarel expire a few minutes after they are 

 brought on shore. From hence this tribe seems possessed of powers 

 that other fishes are wholly deprived of; they can remain continually 

 nder water, without ever taking breath ; while they can venture their 

 heads above the deep, and continue 'for hours out of their native ele- 

 ment. 



We observed, in a former chapter, that spinous fishes have not, or 

 at least appear not to have, externally any instruments of generation. 

 It is very different with those of the cartilaginous kind, for the male 

 always has these instruments double. The fish of this tribe are not 

 unfrequently seen to copulate ; and their manner is belly to belly, 

 such as may naturally be expected from animals whose parts of ge- 

 neration are placed forward. They in general choose colder seasons 

 and situations than other fish for propagating their kind ; and many 

 of them bring forth in the midst of winter. 



The same duplicity of character which marks their general confor- 

 mation, obtains also with regard to their bringing forth. Some bring 

 forth their young alive; and some bring forth eggs, which are after- 

 ward brought to maturity. In all, however, the manner of gestation 

 is nearly the same; for upon dissection, it is ever found, that the 

 young, while in the body, continue in the egg till a very little time 

 before they are excluded; these eggs they may properly be said to 

 hatch within their body ; and as soon as their young quit the shell, 

 they begin to quit the womb also. Unlike to quadrupeds, or the ceta- 

 ceous tribes, that quit the egg state a few days after their first con- 

 ception, and continue in the womb several months after, these conti- 

 nue in the body of the female, in their egg state, for weeks togethe. ; 

 and the eggs are found linked together by a membrane, from which, 

 when the foetus gets free, it continues but a very short time till it de- 

 livers itself from its confinement in the womb. The eggs themselves 

 consist of a white and a yolk, and have a substance, instead of shell, 

 that aptly may be compared to softened horn. These, as I observed, 

 are sometimes hatched in the womb, as in the shark and ray kinds; 

 and they are sometimes excluded, as in the sturgeon, before the ani- 

 mal comes to its time of disengaging. Thus we see that there seems 

 very little difference between the viviparous and the oviparous kinds, 

 in this class of fishes ; the one hatch their eggs in the womb, and the 

 young continue no long time there; the others exclude their eggs be- 

 fore hatching, and leave it to time and accident to bring their young 

 to maturity. 



