284 Mr. Home on the Teredo Gigantea 
passing round its lower end, and up again on the opposite 
side. It is then continued on one side of the oesophagus nearly 
as high as the mouth, where it is reflected over the middle 
tendon of the digastric muscle of the boring shells, and runs 
along the back of the animal, till it terminates in the small 
tube, through which it empties its contents. 
The testicles are two long glandular substances, one on 
each side of the stomach, of a white colour, and granulated 
structure. From each of them a duct passes to the ovaria, 
which lie between the two breathing organs. The ducts run 
upon their outer edge, and terminate near the base of the 
small tube, and in this way the eggs are impregnated, before 
they pass out at that orifice. 
In the worms examined in February, from Sheerness, the 
testicles were small, and no appearance of ovaria could be 
seen : but in some specimens from the Hunterian Museum 
the testicles were much fuller: and the ovaria formed two 
distinct longitudinal ridges ; which, when examined in the 
microscope, contained innumerable small eggs ; when the ovaria 
are empty, there is nothing to be found between the two 
breathing organs but the small seminal vessels. 
Sellius mentions that the teredo navalis has its ovaria full 
of eggs, in the spring and summer : that they are met with 
as late as December, but those teredines, which he examined 
in February had their ovaria flaccid and empty. 
The heart is situated on the back of the animal ; in the 
middle between the mouth and the lower end of the stomach. 
It consists of two auricles, composed of a thin dark-coloured 
membrane ; these open by contracted valvular orifices into 
two white strong tubes, which unite and form the ventricle. 
