ALIMENTARY AND RESPIRATORY SYSTEMS. 463 
called cerebral vesicle, which in the larva communicates 
with the exterior by a pore (the neuropore). From the 
nerve-cord there arise two sets of nerves, dorsal and ventral. 
Of these the two anterior pairs of dorsal nerves, which go to 
the pre-oral hood, are called cranial, and do not correspond 
to the myotomes. Behind these a pair of dorsal nerves 
arise at each myotome, but, as is the case with most of the 
other segmentally arranged parts of the lancelet, the 
members of a pair are not directly opposite to one another. 
The ventral nerves are absent in the region of the two first 
pairs of dorsals, and behind this they divide up into many 
minute fibres just as they leave the nerve-cord. The two 
sets of nerves are compared respectively to the single-rooted 
sensory dorsal nerves, and to the many-rooted motor ventral 
nerves of higher Vertebrates. But the dorsal nerves of 
Amphioxus supply the transverse muscles as well as the 
skin, so that they must be partly motor. Furthermore, 
there is no connection between the two sets, and the dorsal 
nerves have no ganglia, except in so far as these are repre- 
sented by aggregations of nerve nuclei. Nor are there any 
sympathetic ganglia. 
The nervous system of the lancelet is thus very divergent from what 
is typical for Vertebrates :—(1) A brain is almost undeveloped ; (2) the 
ventral roots far outnumber the dorsal roots; (3) the two sets of roots 
do not unite ; (4) the dorsal nerves are partly motor; (5) there are no 
spinal ganglia ; (6) there are no sympathetic ganglia. 
The anterior region of the nerve-cord exhibits some histological dis- 
tinctiveness ; and with it the following structures are associated :— 
(a) Slightly to the left side there is a ciliated pit, often called 
olfactory. It arises from an ectodermic invagination in the position of 
the neuropore or original anterior opening of the nerve-cord. Below 
this there is a minute diverticulum from the front dorsal wall of the 
nerve-cord. 
(6) At the end of the nerve-cord there is » pigment spot, sometimes 
called an eye-spot. There are no true eyes, but numerous regularly 
arranged pigment spots on each side of the spinal cord appear to he 
optic. 
(c) On the roof of the mouth there opens a small sac, the pre-oral pit, 
which may have a tasting or smelling function. 
It is likely that the most important sensory structures of the adult are 
the sensitive cells of the epidermis. The feeble development of sense 
organs may be associated with the almost sedentary habit. 
Alimentary and respiratory systems.—The true mouth 
lies within the projecting pre-oral hood. It is surrounded 
