128 



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 

 Vertebrate 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, 



FIG. SO. 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 ; dil, dorsal dila- 

 tation ; e, eye-spot ; np, neuropore ; olf, olfactory pit ; 7 and //, 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- 



