Fishes. 2545 



of the delicate cartilaginous rays upon which the membrane forming the caudal fin 

 was spread. In its course it gave off numerous branches termed intercostal, to 

 supply the ribs and their muscles, and then another large branch termed cceliac, 

 which is distributed to the rudimentary stomach and alimentary canal ; the principal 

 trunk given off, being that which divides into numerous branches to supply the right 

 side of the vitellicle, and communicates with the veins on its left side, which ulti- 

 mately join and form the large vein termed mesenteric. Another large trunk is given 

 off from the aorta, and bends downwards nearly at right angles, and suddenly turns 

 back and empties itself into one or other of the large branches which go to form 

 the caval vein. The intercostal arteries are given off in pairs; one pair to supply 

 each rudimentary vertebra, and the adjoining muscular and cutaneous parts ; these 

 terminate in the vena cava. In the caudal region, the aorta sends off intervertebral 

 branches, which pass upwards and downwards on each side, and supply the muscular 

 substance of the tail. The vena cava commences in the tail. The capillary branches 

 of the aorta, forming loops at the extremities of the cartilaginous rays, unite into 

 three or four large branches, which terminate in a vein which runs in a canal imme- 

 diately beneath the aorta, receives branches from the intercostal, and all the other 

 arteries in the abdomen, and] ultimately terminates in the auricle. Immediately at 

 the spot in which the caudal veins terminate, may be observed a pulsating cavity 

 or heart, whose office is clearly that of assisting the flow of blood towards the 

 abdomen ; the credit of the discovery of this organ in the eel is given to Dr. Marshall 

 Hall, but it will be clear from the following description, that it was known to 

 Leuwenhoeck. 



" When I fixed these small eels before the glass, and fixed my eye upon the fin 

 near the tail, I saw with greater admiration than ever I did in my life before, the 

 circulation of the blood, and that in so many sundry places ; so that if I should deli- 

 neate the little space composed thereof, it would not seem credible to most men : 

 and when I came to contemplate the end of the bone, I saw, that very near to the 

 jointing of the last joint, many very small veins did meet together, and make them one 

 great one, when I took it for granted that there was a valvula, for there was a strange 

 and quick pushing forwards, after such a manner, as if we saw our blood pushed for- 

 ward in an artery before our eyes ; nay, this pushing was so quick one after the other, 

 that with our mouth we could not so quickly pronounce one syllable after another ; in 

 short, this contemplation did far exceed all the strange and pleasant ones, that ever 

 mine eye did behold before." 



In the early stage, the pulsations of the heart could be distinctly observed in front 

 of the vitellicle, a little red blood could likewise be seen in the parts about to become 

 the gills ; this increased in size day after day, until at last capillary loops were visible 

 in the same situations ; these in some parts gave off small branches from their sides ; 

 the deposit of pigment about this time, which was when they were about two weeks 

 old, was so abundant, that all further trace of vessels into the gills was lost. Whilst 

 these changes were going on in the blood vessels, corresponding changes were also 

 taking place in the vitellicle ; the arteries distributed upon its outer surface became 

 smaller and paler, and their circulation less rapid ; the same thing occurred in the 

 veins ; the vitellicle itself was altered in shape, becoming more elongated, and its coats 

 more opaque from a deposition of dark-coloured matter over the membrana vitelli, 

 and from the thickening of the cuticle which was gradually extending over it. When 

 the author first saw the vitellicle, he imagined, that after it had afforded to the little 



