1898) ANNULATH ANCESTRY OF THE VERTEBRATA 21 
to ensure the purity of the food have developed a beautiful variety 
of sieves and strainers to prevent any solid matter from passing into 
the alimentary canal. With this purely liquid food, they manage 
to distend themselves almost to bursting, by a kind of force-pump 
action of the oesophagus.. The effect of this distension of the 
alimentary canal upon itself and upon the other organs of the body 
can be made to explain all the more important structural pecu- 
liarities and variations in the Arachnida. No single organ or 
assemblage of organs has remained unaffected ; all have had to adapt 
themselves or protect themselves. The whole form of the body has 
been changed, limbs have aborted, and the respiratory and circulatory 
systems have been profoundly modified. So obvious is this to any 
one who studies the group from this point of view, that we are quite 
justified in postulating modifications and adaptations of the organisa- 
tion of our assumed hirudinean ancestor, not only to the full-meal 
condition of its alimentary canal, but also to its more constant load 
of solid lumps. 
In the Arachnida, one necessary precaution was the protection 
of the muscular and nervous apparatus against temporary incapacity 
due to the distension of the alimentary system. This has been 
brought about by a division of labour, one region of the body under- 
taking the animal (locomotory and sensory) functions, the other the 
vegetative and digestive functions. The arachnidan body accordingly 
is divided transversely by a narrow waist or diaphragm ; the alimen- 
tary canal in the posterior division can be distended to its utmost 
limit without pressing at all on the anterior part. The question as 
to which region should be the animal and which the vegetative was 
naturally settled by the fact that the muscular apparatus of the 
jaws and of the capturing limbs, and the ganglia of the great sen- 
sory organs were already at the anterior end of the body. 
Now I assume—and in this assumption lies my new reading of 
the facts—that our supposed hirudinean had to undergo modifications 
for precisely the same end, viz, the protection of the locomotor 
functions from the alimentary. I again suggest that a division of 
labour took place, the body dividing not transversely, as in the 
Arachnida, but longitudinally, 7.c., into a dorsal and a ventral half ; 
and then, that the dorsal half had to protect itself from the 
ventral. 
We will deal first of all with this assumed division of labour. 
The weight of the distended abdomen pressing downwards upon the 
primitive ventral nerve-cord and muscles would seriously affect 
their working ; while, as some compensation for this loss of power, 
the dorsal neuro-muscular system, on which at any rate the 
pressure due to gravitation did not act, would be free to fulfil its 
functions, and thus able to develop in order to meet the greater strains 
