I12 THE ORIGIN OF VERTEBRATES 
a bulging attached ventrally to the extremity of the narrow tube of 
the optic diverticulum. In Fig. 47, A, I reproduce this figure of 
Scott, and by the side of it, Fig. 47, B, I have represented the origin 
of the vertebrate eye as I believe it to have occurred. 
We see, then, this very striking fact, that in the most primitive 
of the Crustacea, not only are there two anterior diverticula of the 
gut, but also the retinal ganglion of the lateral eye is in specially 
close connection with the end of the diverticulum on each side. In 
fact, we find in the nearest living representative of the trilobites a 
retina and retinal ganglion and optic nerve, closely resembling that 
of the vertebrate, in close connection with an epithelial tube which 
has nothing to do with the organ of sight, but is one of a pair of 
anterior gut-diverticula. It is impossible to obtain more decisive 
evidence that the trilobites possessed a pair of gut-diverticula sur- 
rounded to a greater or less extent by the retina and optic nerve of 
each lateral eye. 
Such anterior diverticula are commonly found in the lower 
Crustacea ; they are usually known by the name of liver-diverticula, 
but as they take no part in digestion, and, on the contrary, represent 
that part of the gut which is most active in absorption, the term 
liver is not appropriate, and it is therefore better to call them simply 
the pair of anterior diverticula. Our knowledge of their function in 
Daphnia is given in a paper by Hardy and M‘Dougall, which does 
not appear to be widely known. Hardy succeeded in feeding Daphnia 
with yolk of egg in which carmine grains were mixed, and was abie 
in the living animal to watch the whole process of deglutition, 
digestion, and absorption. The food, which is made into a bolus, is 
moved down to the middle region of the gut, and there digestion 
takes place. Then by an antiperistaltic movement the more fluid 
products of the digestion-process are sent right forward into the two 
anterior diverticula, where the single layer of columnar cells lining 
these diverticula absorbs these products, the cells becoming thickly 
studded with fat-drops after a feed of yolk of ego. The carmine 
particles, which were driven forward with the proteid- and fat- 
particles, are not absorbed, but are at intervals driven back by con- 
tractions of the anterior diverticula to the middle region of the gut. 
These observations prove most clearly that the anterior diver- 
ticula have a special nutrient function, being the main channels by 
which new nutrient material is brought into the body, and, as 
