148 THE LANCELET 



6. The Nervous System. 



This may be divided into the following two parts : — 



(a) The central nervous system, consisting of a dorsal spinal cord lying above 

 the notochord and almost as long as it. It is enclosed in a sheath of 

 connective tissue. The central canal of the spinal cord expands an- 

 teriorly into a cerebral vesicle. From the anterior end of this vesicle 

 arises a small diverticulum connected with an ectodermal olfactory pit. 

 (6) The peripheral nervous system, consisting of two series of nerves arising 

 from the spinal cord, viz. : 



(i) The dorsal sensory nerves, with a single root. The first two 

 pairs may be regarded as cranial nerves. All these nerves 

 supply the skin, 

 (ii) The ventral motor nerves, with multiple roots, supplying the 

 myotomes. 

 Notice the pigment spots, one (the " eye ") in the vesicle wall at the front 

 end of the cord, and others at intervals along the cord. 



Transverse Sections through the Body. 



1. In a section taken through AB (the posterior part of the pharynx) in 

 Fig. 49 the following parts may be seen : — 



(a) The section has, roughly, the shape of an isosceles triangle, the apex being 



formed by the dorsal fin and the base by the ventral surface, between 

 the metapleural folds. The greater part of the interior is occupied by 

 the atrial chamber. 



(b) The body is covered by a single layer of epithelial cells, forming the 



epidermis. Underlying the epidermis is the dermis, formed mainly of 

 soft connective tissue. 



(c) The myotomes of the lateral muscles, separated by partitions of con- 



nective tissue, the myocommas ; and the ventral or transverse muscles, 

 extending across the ventral surface of the body in the floor of the 

 atrial chamber. 



(d) The fin ray of the dorsal fin. 



{e) The spinal cord, situated above the notochord. 



(/) The notochord, beneath the spinal cord. 



(g) The pharjTix, in the middle of the atrial chamber. Its side walls appear 

 interrupted owing to the section cutting through a series of oblique 

 gill-slits. The dorsal wall of the pharynx is folded to form the epi- 

 pharyngeal groove ; and the ventral wall is folded to form the 

 endostyle. 



(h) The liver, lying on the right side of the pharynx. 



(i) The gonads attached to the lateral walls of the atrial chamber. 



(j) The metapleural folds, with a canal or lymph space in each. 



(k) The coelom, which is divided into : — 



(i) A portion surrounding the liver. 



(ii) The dorsal ccelomic canals, a pair of spaces at the sides of the 

 epibranchial groove. 



