288 THE ZOOLOGIST. 
a dorsal rod, the microscopic structure of which is precisely 
comparable to that of young Lampreys, or young Sharks, and 
which he has no hesitation in calling a notochord; he finds that 
the central nervous system is dorsal and forms a hollow tube, 
and as to the gill-apparatus he tells us that it may be well com- 
pared to that of Amphioxus; just as in the Lancelet, the gill- 
slits increase in number from before backwards during life. 
There are other points of resemblance, but those which I have 
indicated are sufficient for our purpose to-day. 
These observations appear to justify the inclusion of Balano- | 
glossus in that great trunk of the animal tree which is called the 
Chordata, which will then, as Mr. Bateson suggests, consist of 
four groups :— 
1. Hemichordata (Enteropneusti). 
2. Urochordata (Ascidians). 
3. Cephalochordata (Amphioxus). 
4. Vertebrata. 
Having now brought the origin of the vertebrate stock so far 
down in the ancestral tree, we may as well see if we cannot trace 
it yet a little further. If the views of Metschnikoff are just, 
Balanoglossus is most closely allied to the Echinodermata, or 
Starfishes and Sea-Urchins; this view, however, is too problem- 
atical for us to be able to regard it as an acquired fact in - 
Zoology, and, so far as general audiences are concerned, it can 
only be mentioned as affording an opportunity for repeating the 
humorous remark of my friend and colleague Prof. Martin 
Duncan, that the highest Vertebrate is always an Urchin in his 
earlier days. 
We can, however, trace the Vertebrate far and deep into the 
vermian mob. One division of the lowest groups of worms, the 
Nemertinea, are remarkable for the possession of a proboscis 
which can be protruded from an orifice in front of the mouth, 
and be withdrawn into a sheath which lies above the digestive 
canal; now this sheath lies in the dorsal middle line, or, in other 
words, occupies an exactly similar position to the notochord, and, 
so far as is known, in developmental history the notochord and 
the sheath appear to have much in common. Let this be the 
first point. 
Nearly all the Vertebrata have on the lower surface of their 
brains a body which, in the highest group of all,—or that of the 
