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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 blood- 

 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 sinks into 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 a true 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, 



