258 Sir Everard Home on the structure of the 
as in the lamprey, only the cartilages of the thorax are so 
weak as to appear like ligaments, and the pericardium is 
membranous. 
In an animal brought from the South SeasT>y Sir Joseph 
Banks, intermediate between the lamprey and myxine, but 
differing so much from both as to form a distinct genus, the 
respiratory organs resemble those of the lamprey in the 
number of the external openings, and the number of bags, 
but these organs and many other parts differ in the following 
particulars, in which they agree with those of the myxine. 
There is no appearance whatever of thorax, nor is the peri- 
cardium cartilaginous ; the bags are flattened spheres placed 
perpendicularly, their cavities are small, their coats elastic, 
and the internal orifices communicate directly with the oeso- 
phagus, which is small. The oesophagus does not terminate 
in a valvular slit, but in a loose transverse membranous fold ; 
there are two rows of teeth on each side of the tongue, bent 
downwards, long, and pointed. There is a posterior nostril, 
and an appearance resembling an uvula. There is a. gall 
bladder, a row of large mucous glands on each side of the 
belly, and there is a mesentery to the intestine. 
In the myxine, the respiratory organs differ from those 
last described, in there being only two external openings, and 
six lateral bags on each side, to which there are six tubes from 
each of the openings, and, close to the left external opening, 
there is one which passes directly into the oesophagus, the 
gall duct projects into the intestine. 
In the aphrodita aculeata of Linn^us, the respiratory organs 
as well as the other viscera, differ in many respects from those 
of all the other animals of that tribe. There are thirty-two open- 
