KUTCHIN. — PERIPHERAL NERVOUS SYSTEM OF AMPHIOXUS. 591 



a short distance into two branches, Avhich reach the vekim a short 

 distance ventral to the muscle border. These two latter branches 

 unite in the velar muscle, and the band-like nerve thus formed may be 

 followed in almost the entire circumference of the ring muscle, although 

 its size varies considerably in different portions. 



The smaller branch {^), leaving the place of anastomosis between 

 the branches of ner\-es V and VI, takes a general course backAvard 

 over the inner surface of the trunk muscles, running for a short dis- 

 tance dorsal to the larger branch. Near the \-elum these two nerve 

 branches (a, j3) unite, forming a short, thick trunk. This trunk divides 

 into two short, diverging branches of equal size. The more dorsal 

 branch (6') enters the velum about the width of two myomeres dor- 

 sal to the ventral muscle border of the left side, and is continued 

 dorsad in the circular muscle of the velum as a slightly irregular, 

 thick band. This band curves over to the right side of the velum, 

 and breaks up into the velar plexus. The more ventral (rt) of these 

 short, diverging branches continues ventrad in the ring muscle of the 

 velum, losing its band-like character at the base of the fifth tentacle 

 ventral to the border of the trunk muscles. These band-like nerves 

 lie chiefly near the bases of the skeletal portions of the velar tentacles, 

 but do not form a continuous "band nerve." The velar plexus lies, 

 for the most part, between these thick, flat nerves and the free portions 

 of the tentacles. A comparatixely large branch passes to each tentacle 

 obliquely from one of the band-like nerAes; tliis oblique deflection is 

 usually ventralward, but may vary. A single large tentacle in the 

 mid-ventral region receives two main branches, one from the right 

 side, and one from the left. The velar plexus is completed by numer- 

 ous small branches, which anastomose to form an irregular, loose- 

 meshed network between the band nerves of the circular muscle and 

 their large branches which supply the tentacles. Toward the more 

 external border of this plexus many apparently free-ending small 

 nerve branches are visible. These may terminate in some form in the 

 velar epithelium, or may possibly join a finer unimpregnated plexus. 



The nerves of the velar plexus generally exhibit a flattened, granu- 

 lar appearance when impregnated with methylene blue, and their 

 outlines do not present the clear-cut aspect noticeable in cutaneous 

 branchlets of dorsal nerves. Their structure calls to mind that of the 

 nerve branches forming the inner plexus of the mouth border, and the 

 inner abdominal plexus. Apparent oval nuclei, or nucleated cells, 

 may be frequently observed in the course of the finer branches of the' 

 velar plexus (PI. 2, Fig. 9). 



