11 
absolutely unique. ‘These four great structural features are, first 
the primitive backbone or notochord ; second the throat perforated 
by gill slits; third, the tubular nerve centre or spinal chord and 
brain, placed along the back; and lastly, and perhaps most dis- 
tinctive and clinching as an evidence of affinity, the myelonic or 
cerebral eye.”’ (Ray Lankester). 
Without going into detail concerning these four organs, I may 
yet draw your attention to one or two points in its structure. Its 
outward appearance is well shown here, and we have also a 
diagrammatic representation of its internal arrangements. The 
upper aperture or mouth leads into a cavity or chamber of con- 
siderable size, the walls of which are a close network of blgod- 
vessels, full of perforations or meshes opening into a second cavity 
which communicates freely outwards, by the lower, or atrial 
opening. A constant stream of water flows in at the mouth, 
through these meshes, and out by the atrium, exactly as water 
passes into the mouth of a fish, and out by the gill openings. And 
for the very same purpose, the network answering to the gills of 
the fish, i.e., effecting the purification of the blood. The food 
which enters with the water passes downwards into the alimentary 
canal, which likewise opens into the atrium. Thus we see, there 
are mysteries wrapped up even in this leathery Sea Squirt, and 
processes are at work in it exactly corresponding to those in our 
own system, proclaiming a common kinship. But now, this fixed and 
rooted organism, which has just about as much relation to the world 
outside itself as that which formerly characterized the Chinese 
nation, produces eggs which give rise to a creature totally unlike 
the parent,—a creature which occupies a place in the animal 
kingdom higher than the mature Ascidian, and which, as I have 
to show you, has actually to degenerate in its structure by two or 
three stages until it sinksinto maturity. Soon after emerging from 
the egg the infant Squirt has a tadpole form; notice its resemblance 
to that of the frog, though of course the latter is much larger. 
There is the mouth, there is one eye, only one, but that is more 
than its parent had; there is a nervous system prolonged behind, 
and in the tail the structure known as the notochord, which 
occurs otherwise only in vertebrated animals. With them as with 
this, it occurs only in an early embryonic stage, but is soon replaced 
by atrue spinal column. This larva is a free swimming creature, 
actively roaming about and seeking its own food. Having within - 
it those four elements of development into the same division as that 
which we ourselves occupy, how is it that it fails to rise beyond 
one or two steps? How is it that it developes upwards to a 
certain point, and then commences a downward career? Degrada- 
tion it undoubtedly is, for it loses the solitary organ of sight, 
