128 CEPHALOCHORDATA CHAP. 
supplying the integument as well as the transverse muscles, so 
as to be sensory as well as motor, and a ventral series arising 
each by a number of roots (Fig. 81) and wholly motor in 
function, as they supply only the myotomes. These two series 
may be compared to the dorsal and ventral roots which in the 
Vertebrata join to form a mixed spinal nerve. 
In addition to ordinary small nerve cells the central nervous 
system contains certain large nerve cells with very long processes, 
Pia. 80.—Branchiostoma lanceolatum. A, brain and cerebral nerves of a young speci- 
men; B, transverse section through neuropore; C, behind cerebral vesicle; D 
through dorsal dilatation. ch, Notochord ; cv, cerebral vesicle ; di/, dorsal dila- 
tation ; e, eye-spot ; mp, neuropore ; olf, olfactory pit; J and J//, cranial nerves. 
(From Willey, after Hatschek.) 
the “ giant fibres,” which extend through the greater part of the 
length of the spinal cord. No trace of a sympathetic nervous 
system has been found. 
The Sense-Organs connected with the nervous system are 
few and simple. There are sensory cells in the ectoderm, on the 
margin of the velum,on the velar tentacles, and especially in 
clumps on papillae of the cirri around the mouth, which are 
probably tactile. In the roof of the oral hood there is a sensory 
structure, the “ groove of Hatschek,” which is supposed to be an 
organ of taste. The olfactory pit alluded to above opens ex- 
ternally on the left-hand side of the snout. It is ciliated 
internally and leads to the so-called olfactory lobe, an antero- 
