VERTEBRATES FROM A CRUSTACEAN-LIKE ANCESTOR. 381 



as the pineal ej^e was easily recognisable and of a Crustacean 

 type, and that the proportion of nervous material to non- 

 nervous approached nearer to the proportion found in the 

 Crustacean than in the higher Vertebrates. 



In addition, we should expect to find the vestiges of other 

 Crustacean organs which are connected with the alimentary 

 canal, especially that large and important organ the so-called 

 liver ; and, indeed, seeing that the so-called liver represents the 

 whole of the digestive glandular apparatus of the alimentary 

 canal of the Crustacean, the cephalic stomach and straight in- 

 testine being simple epithelial structures for the purpose 

 respectively of holding food, perhaps also of absorbing it, and 

 of passing out the indigestible residue, it follows that any 

 theory which compares the simple epithelial non-nervous tube 

 of the central nervous system of the Vertebrate with the non- 

 glandular epithelial stomach and intestine of a Crustacean is 

 imperfect, unless some explanation is given of the fate of the 

 large glandular liver Avhich must have existed in the cephalic 

 region of the ancestor of the Vertebrate if that ancestor was 

 formed after the Crustacean type. 



Conversely, the theory which I have put forward is im- 

 mensely strengthened if the examination of the lowest type of 

 Vertebrate brain brings to light the vestiges of a large glan- 

 dular organ, which bears the same anatomical relation to the 

 ventricular cavity of the brain as the so-called liver bears to 

 the cephalic stomach of the Crustacean. 



I have chosen the Ammocostes stage of Petromyzon as the 

 subject of investigation because it undoubtedly represents a 

 very primitive form among living Vertebrates, and also because 

 it is easy to obtain in large quantities at diflerent ages, and 

 will live well in the laboratory : all my specimens were ob- 

 tained by me at Hind Head, in Surrey — the first batch, to the 

 number of about forty, in the beginning of September of 1888; 

 and the second batch, amounting to 100, at the end of October, 

 1889. They were all Ammocoetes of Petromyzon Plaueri, 

 and varied in size from 25 mm. in length up to full-grown 

 specimens of 130 mm. or more. 



VOL. XSXI, PART III. — NEW SEE. C C 



