THE ANCESTRY OF THE VERTEBRATES 533 



During this metamorphosis, which is a regressive one, the 

 tail, with its notochord and segmented muscles, becomes lost, 

 and the central nervous system becomes much reduced. The 

 posterior end of the intestine connects with the cloacal cham- 

 ber and the adult relationships are gained. 



From this sketch of the organization and metamorphosis 

 of the tunicates it is evident that the group is somewhat closely 

 related to Amphioxus, and hence to the vertebrates, but that, 

 since the time of the common ancestor, the Tunicata have fol- 

 lowed for a long distance a divergent road of special adapta- 

 tion, which, although it has allowed them to continue exist- 

 ence, has been one of degeneracy and loss and has masked 

 their true relationships. The ancestor that we here seek is 

 better seen in the larva than in the adult, and we may believe 

 that there once existed an adult animal with attributes like 

 that of the tunicate larva of the present day, and that this ani- 

 mal was the direct ancestor of that group of which Amphioxus 

 is now the only living representative. 



Is there now any record of the history of vertebrate descent 

 back of the tunicate ancestor ? Is there any other invertebrate 

 animal possessed of gill-slits, a notochord, and a dorsal nervous 

 system ? And as an answer to this, a very incomplete and un- 

 certain one at best, there is only a single animal form, though 

 represented by several closely allied species, an animal as un- 

 promising in its exterior, and here, perhaps, almost as much 

 so in its interior also, as those hitherto considered. This 

 form is Balanoglossus, a marine worm that lives in self-con- 

 structed tubes of sand between tide-waters, and, like Amphi- 

 oxus, has a wide distribution. Externally the animal is worm- 

 like and is possessed of four body regions, a conical proboscis, 

 a collar with a free anterior edge, a flattened gill region and a 

 cylindrical posterior part. The mouth is situated ventrally, 

 immediately beneath the edge of the collar, and receives the 

 sand mixed with nutrient material as it becomes unearthed 

 by the burrowing action of the proboscis. The pharynx is 

 quite extended and communicates directly with the exterior 



