828 Reptiles. 



In the early state of the tadpole, it is well known they have exter- 

 nal branchial tufts, which perform the function of respiration ; these 

 disappear, and gills for aquatic respiration take their place. Their 

 appearance and disappearance depend on the temperature in which 

 the tadpoles are kept. They appear before the young escape from 

 the jelly-like envelope. In the ova procured in December, these 

 tufts first appeared on the fourth and fifth day ; and those obtained in 

 January, with a higher temperature, on the second day after the ova 

 are shed. In the disappearance of these tufts, it is a remarkable fact, 

 that in all I examined, the tufts on the right side disappeared first, 

 and in many instances, one day earlier than those on the left. 



Connected with the temporary gills, there is a remarkable analogy 

 existing between the cartilaginous fishes, the sharks and rays, and 

 the frog. Temporary filaments, similar to those mentioned above, 

 have been observed by Professor Owen, in the blue shark ; in the 

 thornback, by Dr. Allen Thompson ; and in the torpedo, by Dr. John 

 Davey ; and I have examined them in the rough hound (Sq. Canicula), 

 nurse hound (Sq. Catulus), miller-dog (Sq. galeus), smooth hound 

 {Sq. mastelus), picked dog {Sq. acanthias), and many kinds of skates 

 and rays, and in all, the filaments were long, slender, simple, and 

 unbranched, traversed by a single reflected vessel. These were very 

 long when the embryo was about one inch, or one and a half inch long ; 

 at this time they were about three quarters of an inch long, but they 

 gradually got shorter, and finally disappeared before birth. It is 

 familiarly known, that the gills of the sharks and rays are unlike the 

 gills of other fish, free : on the contrary, they are, in a great measure, 

 fixed and contained in chambers, which open externally by five ori- 

 fices. In each chamber there are two gills, one attached to the ante- 

 rior wall, and the other to the posterior. On the anterior only of 

 these are the filaments found, and the vessels which traverse them 

 are the continuations of the true vessels of the gills. This analogical 

 structure is curious. In the land newt these appendages disappear 

 before birth, in the same manner as in the sharks, but in the frog they 

 remain for a short time after birth, while in the triton, or water newt, 

 they remain till the animals are half grown. 



R. Q. Couch. 

 Penzance, December, 1844. 



