THE ANCESTRY OF THE CHOBDATA. 97 



The Affinities of the Chordata. 



Having thus examined the history of those organs which the 

 morphology of Balanoglossus enables us to trace, let us 

 consider the relations of Chordata (1) to other groups, (2) to 

 each other. 



Of the Echinodermata. — Unlikely though it may seem, 

 if any reliance can be placed on the characters of pelagic 

 larvae, we must assume some affinity between Echinodermata 

 and Chordata, for Tornaria is not very like, but practically 

 identical with, Bipinnaria. The case is like that of Mollusca, 

 which may be supposed to be allied to Annelids, as is indicated 

 by the trochosphere larva. 



Of the Nemertines. — So much has been said by previous 

 writers as to the Chordate affinities of Nemertines that the 

 subject cannot be omitted. The suggested homology of the 

 nervous system has already been dismissed. Hubrecht has 

 further suggested (1) that the notochord is homologous with 

 the proboscis sheath of Nemertines, (2) that the cephalic 

 pits are gill-slits, (3) that the proboscis is the pituitary 

 body. 



With regard to (1), what can be adduced from a study of 

 Enteropneusta seems rather to be opposed to this view. If 

 this were true, the notochord must have arisen in some such 

 body as that of a Rhabdocoel, into the wall of the endoderm 

 of which a prseoral lobe could be invaginated, rather than as a 

 hard thickening which is constricted off to form a lumen. 

 Into the free end of such a structure it is impossible to con- 

 ceive the invagination of a proboscis, which is what Hubrecht's 

 suggestion seems to require. All that can be said is that the 

 notochord of Balanoglossus suggests that it arose as a support- 

 ing structure and not as a modification of something else. 



But supposing the larva in Stage G to represent a phyloge- 

 netic phase, several points of Nemertine anatomy can be 

 derived from it. At this stage it has one pair of gill-slits, a 

 short nerve-cord, one median anterior mesoblastic pouch, and 

 two pairs of posterior pouches. Now, on the hypothesis of 



