ALIMENTAR Y AND RESPIRA TOR Y S YSTEMS. 463 



called cerebral vesicle, which in the larva communicates 

 with the exterior by a pore (the neurppore). 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 : (i) 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. 



(b] At the end of the nerve-cord there is a 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 be 

 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 



