146 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



on one side alternate, like the myomeres, with those on the other 

 side. The first pair have a ventral origin from the extreme front 

 of the nerve-cord, and, together with the second pair, which arise 

 dorsally behind the ventricle supply the parts in front of the 

 mouth, while the third pair supply the mouth-region. These 

 single-rooted nerves are partly sensory, innervating the skin, and 

 partly motor, giving off twigs to the muscles. Their sensory 

 branches divide repeatedly, and their finest ramifications ulti- 

 mately unite together to form a delicate nerve-plexus below the 

 epidermis. Such a nerve-plexus, investing the whole body, is 

 characteristic of certain non-chordates, and must be regarded here 

 as a primitive feature inherited from non-chordate ancestors. 



The multiple-rooted nerves all arise from the ventral side of the 

 nerve-cord for the greater part of its extent, alternating with the 

 dorsal nerves. Each of them consists of a longitudinal series of 

 slender bundles (roots) which remain independent of one another, 

 and branch out in the adjacent muscles. These nerves, therefore, 

 are of motor nature. 



10. The sense-organs of Amphioxus are ill-developed and for 

 the most part of problematical nature. 



A considerable number of the epidermal cells, especially in the 

 anterior part of the body, probably serve as tactile organs. These 

 cells are comparatively slender, continuous internally with sensory 

 nerve-fibres and produced externally into a stiff process. 



A gustatory function has been ascribed to the following struc- 

 tures : ( 1 ) Small aggregates of sense-cells, similar to those 

 described in the last paragraph and situated upon the buccal 

 cirri. The external processes of each group form a conical pro- 

 jection. (2) Circlets of sense-cells occurring in the velar tentacles. 

 (3) A sac lined by sensory epithelium and opening on the left 

 side of the roof of the buccal cavity. 



It is exceeding doubtful whether the so-called olfactory pit, 

 situated on the left side of the head, has anything to do with 

 smell. It is a depression lined with ciliated epithelium and 

 communicating with the ventricle. 



A pigmented mass imbedded in the front wall of the ventricle 

 has received the name of eye. 



