224 



COMPARATIVE ANATOMY 



CHAP. 



branches pass into each other dorsally, and thus form in the body wall 



numerous consecutive nerve rings. 



The nervous system of the Myzostomidcn (Fig. 149) is now very 



exactly known. It consists of an oesophageal ring surrounding the base 



of the pharynx, whose dorsal lateral 

 portions are somewhat thickened, and 

 so represent a weakly developed brain. 

 Close to the oesophageal ring lie gangli- 

 onic cells. Further forward in the 

 pharynx a second nerve ring (pharyn- 

 geal ring) is found, which is connected 

 with the oesophageal ring by several 

 longitudinal nerves, and gives off nerves 

 to the tentacles at the free anterior 

 edge of the pharynx. On the ventral 

 side, under the integument, lies a large, 

 elongated, ganglionic mass, from whose 

 anterior end the two commissures 

 (limbs) of the oesophageal ring arise. 

 *=s ^ , /CSISil ' ^^ The ganglionic mass represents a ven- 



— ^^ |**'.aa, ';^( ^^.^j chord which consists of several 



(jjrobably 6) fused pairs of ganglia, and 

 ^jfTft in which an intermediate nerve is to 

 be found. From this ganglionic mass 

 11 alternately stronger and weaker 

 nerves radiate on each side towards the 

 circumference of the disc-shaped body ; 

 these nerves branch in a complicated 

 Pig. i49.-centrai nervous system of manner and innervate especially the 



Mvzostoma, after Nansen. gc, Cerebral i , r .-, t ._i i i 



commissure; pr, pharyngeal nerve ring; mUSCUlature of the parapodia, the hooks, 



hgm, ventral ganglionic mass, witii the the cirri at the edge of the body, and 



proximal part of the diverging nerves ;sr, ^|j integument and its mUSCulature. 



The existence of a sympathetic nervous 

 system also seems probable. 

 Pposopygia. — The nervous system of the Sipunculacea is in 

 many respects closely related to that of the Echiuridce. As there is 

 no long prostomium, the ossophageal ring is also not elongated 

 anteriorly, but forms a simple ring at the anterior end of the enteric 

 canal. This ring in the Priaimlidcn is only a little thickened dorsally, 

 while in the Sipunculidce it enters a well- developed brain. Nerves 

 diverge from the brain to the anterior end of the body and to the 

 tentacles (where the latter are piresent). Nerves also diverge from the 

 oesophageal ring; 2 of these in Sqmnculus supply the intestine, 

 forming a ganglion on each side, while in Friafiulus 4 penetrate the 

 pharynx. The ventral strand, which is covered with ganglionic cells 

 along its whole course, runs in the ventral middle line of the body to 

 its extreme posterior end, where it swells somewhat. In its whole 





oesophageal commissures ; tn 

 nerves ; gz, ganglionic cells. 



tentacle 



