RECENT ADVANCES IN ZOOLOGY. 289 
Mammalia,—becomes actually fused with the brain itself; when 
thus fused it appears to be a downgrowth—or hypophysis—of the 
brain, and it has acquired the distinctive term of the hypophysis 
cerebri. But, although thus connected with the brain, the down- 
growth in question has had a very different history to the great 
mass with which it has finally entered into relation. It was at first 
an upgrowth of the roof of the pit which goes to form the mouth, but 
its stalk becomes gradually nipped out, and the formation of the 
cartilaginous or bony palate separates it from the cavity from 
which it sprung. 
In the lowest of Vertebrates—or in those in which there are 
no true jaws—the so-called Round-mouths, like the Lamprey 
and the Hag, the hypophysis cerebri does not arise directly from 
the roof of the pit which forms the mouth, but appears as an 
independent pit in the front of the mouth. 
Now, as to the Nemertines: the proboscis, which is invested 
by the sheath, ordinarily arises as an ingrowth in front of, or 
independently of, the mouth, just as does the hypophysis cerebri 
in the Lampreys; but in two genera of Nemertines (Akrostomum 
and Malacobdella) it appears as an outgrowth of the roof of the 
mouth—or, in other words, is developed in just the same way as 
the hypophysis cerebri of most Vertebrates. Let these form our 
second sets of facts. 
If we have found some plausibility in the appearance of 
resemblance between the proboscis-sheath and the notochord, it is 
clear that our case will be made very much stronger if we can see 
anything in the Nemertine which will correspond to gill-slits. 
On either side of the anterior end of the body of a Nemer- 
tean worm we observe a structure which, varying somewhat in 
character, may be spoken of as a sac, a furrow, ora groove. The 
sides of this organ are ciliated, and are, therefore, able to set in 
movement any body, such as water, which may enter them; the 
groove is sometimes continued into a canal which passes into 
the substance of the nerve-cells of the brain. These cells are ° 
impregnated with a substance which gives them a reddish-yellow 
colour, and which has been found to be hemoglobin—or that 
substance which gives the reddish-yellow colour to our blood- 
corpuscles, and which is known to be capable of storing up 
oxygen, or of acting as a respiratory pigment. Now the ciliated 
ducts bringing in water bring in afresh supply of oxygen, and 
