112 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 1 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 able 

 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 egg. 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 



