190 Balanoglossus. [zoe 



ticulars in which its structure and development resemble the typi- 

 cal chordate. These are as follows: Of the first class, (i) the posi- 

 tion and origin of the central nervous system; (2) the possession 

 of anotochord; (3) the possession, method of origin and arrange- 

 ment of gill-slits. Of the second class, (i) the origin of the meso- 

 blast — the middle germ layer; (2) the asymmetry of the anterior 

 parts; (3) the opercular fold; (4) the excretory funnels opening 

 into the atrial cavity. It must serve our present purpose to con- 

 sider the three primary features here enumerated; the secondary 

 ones must be passed by with some general statements, merely. 



All vertebrates are characterized by the possession, at least in 

 embryonic life, of a notochord arising from the dorsal portion of the 

 primitive digestive tract, and extending parallel with the long axis 

 of the animal; by the possession of a cerebro-spinal nerve axis that 

 arises from the ectoderm of the dorsal portion of the embryo, and 

 extends parallel with the notochord along its dorsal side; by the 

 possession of paired respiratory organs that arise from the anterior 

 portion of the digestive tube and communicate with the external 

 world, either through the mouth or independently of it; and by the 

 possession of a large median dorsal blood vessel situated between 

 the digestive tract and the notochord, in which the blood flow from 

 before backward. 



To all these fundamental features Balanoglossus certainly presents 

 some remarkable resemblances. The nerve cord arises from the 

 dorsal portion of the ectoderm of the embryo by a process that is 

 quite similar to that by which the same structure arises in many 

 of the fishes, as the lamprey and the bony fishes. However, cer- 

 tain important differences must not be disregarded. 



In all vertebrates the posterior end of the medullary plate — the 

 nerve cord in its early stage — terminates at the blastopore, while in 

 Balanoglossus, it does not extend so far back; in fact the portion of 

 it that seems most nearly to resemble the vertebrate cord is ap- 

 parently confined to the collar, while the anus is situated at the ex- 

 treme posterior end of the animal. Furthermore it seems quite 

 doubtful if the canal, or space that finally appears in the cord of 

 Balanoglossus, is in any sense morphologically comparable to the 

 vertebrate neural canal. 



Again, it is to be observed that in all vertebrates, even including 

 Amphioxus, the nerve cord is encased in a connective-tissue or 



