438 THE ENTEROPNEUSTA. 



ages, which develop side by side with it on the neural surface. It rarely keeps 

 pace with the growth of the body, usually concentrating at the anterior end, 

 around, or close to, the stomodaeum. 



2. The hcemal nerve cordis unpaired, never distinctly segmented, and never 

 associated with important sense organs or appendages. It is the last part of the 

 nervous system to develop, and is associated most intimately with the heart and the 

 haemal and somatic musculature, which are the last parts of the embryonic body 

 to develop. The hasmal cord extended primarily from end to end of the trunk 

 and was united with the neural cord by segmental, circular nerves. It likewise 

 fails to keep pace with the growth of the body; its point of concentration, however, 

 is never in the anterior cephalic region, but in the cardiac and respiratory region, 

 which tends to shift farther and farther back, beyond the oral and thoracic, and 

 finally into the abdominal region. (See Chapter XII.) Its principal connection 

 with the neural system is with the stomodaeal nerve centers at the head end, and 

 with the respiratory neuromeres of the abdominal region. But even the former 

 connection is lost, or greatly reduced in the highest types, leaving only the seg- 

 mental cardiacs of the vagus and respiratory neuromeres as a means of connect- 

 ing the main haemal nerve with the nerve cords on the opposite side of the body. 

 (Limulus). 



The principal parts of the nervous system of the enteropneusta correspond 

 with those of the arachnids as outlined above, the most important point being the 

 identity of the primary neural surface of the arthropod with the principal neural 

 surface of the enteropneusta. In the arthropods, it is true, the medullary plate 

 is on the oral side, while in the enteropneusta it is on the aboral side, but it is the 

 mouth that has changed, not the neural surface with all its fundamental relations. 

 The haemal mouth is a new formation, while the old mouth may still be recog- 

 nized in its proper place in the median depression of the cephalic lobes. (Fig. 

 297, n.st.) The medullary plate of the proboscis represents the remnants of 

 the supra-oesophageal ganglion, and is still connected with the small eye spot, ac, 

 which probably represents the remnants of a parietal eye. (Figs. 296-298.) 



The thoracic neuromeres form the principal part of the nervous system, 

 and at an early period are bodily infolded to form the floor of a canal or tube. 

 The latter usually remains open at either end, and a variable number of vertical 

 canals, or strands of tissue, persist over the median line and appear to mark the 

 point where the medullary folds are imperfectly united (Figs. 296, 297, iv.), 

 recalling the vertical strands of ectoderm between the compacted thoracic neu- 

 romeres in the insects and arachnids. (Figs. 221, 229 and 231.) 



In cross sections the thoracic or collar neuromeres form a thick band of small 

 nerve cells with an underlying layer of "punct substanz." The general appear- 

 ance of the cord is similar to that of a young scorpion, the more so since the nerve 

 cells, for a time, may be arranged in radiating lines around minute cavities (Har- 

 rimania, Ritter) like those so characteristic of the embryonic cords of the arach- 

 nids. (Figs. 15, 16 and 227.) 



