VERTEBRATA ACRANIA. 145 



to the other, can be effected by alternate contraction of the lateral 

 muscles on each side. 



Transverse muscles run across the floor of the atrial cavity, 

 and these no doubt assist in the expulsion of water through the 

 atriopore. 



The muscle-fibres are transversely striated rhomboidal plates. 



9. The nervous system consists of a dorsal nerve-cord and of 

 nerves connected with this. 



The nerve-cord (spinal cord) constituting the central part of 

 the nervous system is a thick- walled tube exactly fitting the 

 neural sheath above the notochord, and extending the greater 

 part of the length of the body. In front, however, it does not 

 reach so far forwards as the notochord, but terminates bluntly 

 just above the front end of the mouth. The cavity of the nerve- 

 cord forms an exceedingly narrow central canal, except anteriorly, 

 where it dilates into a much larger ventricle which opens to the 

 exterior on the left-hand side within the olfactory pit. 



Behind the ventricle, a narrow slit, the dorsal fissure, divides 

 the upper part of the cord, above the central canal, into right and 

 left halves. 



The nerve-cells are grouped round the central canal and dorsal 

 fissure; in a stained section they appear as a deeply-coloured 

 dorso-ventral streak, readily seen under the low power. As in 

 other cases .they are produced into processes by which union 

 with one another and with nerve-fibres is effected. The smallest 

 nerve-cells have but one process (i.e., are unipolar), the larger ones 

 generally have several processes (i.e., are multipolar^). A double 

 longitudinal series of small irregular masses of black pigment- 

 cells is imbedded in the floor of the central canal. The greater 

 part of the nerve-cord is made up of slender nerve-fibres, most of 

 which take a longitudinal direction, while others pass out into 

 the nerves. 



Numerous segmentally-arranged nerves take origin from the 

 nerve-cord and constitute a peripheral nervous system. Most 

 of them correspond to myomeres, being consequently arranged 

 asymmetrically and not in regular pairs. The nerves are of two 

 kinds (1) single-rooted and (2) multiple-rooted. The single- 

 rooted nerves (except the first two) arise from the dorsal surface of 

 the cord as single bundles of nerve-fibres, and the first six of 

 them constitute three regular pairs. Behind this, however, those 

 2 10 



