208 INVERTEBRATE MORPHOLOGY. 



digestive tract occur, which normally are filled with air and 

 are richly supplied with blood-vessels ; they may be respira- 

 tory in function, and have been compared to the swim-bladder 

 of fishes. 



The nervous system is well developed in all Polychseta, 

 and consists of a supravesopliageal ganglionic mass situated in 

 the head segment, frequently presenting a division into sev- 

 eral lobes. From it various nerves arise passing to the an- 

 terior segment, and in addition a strong cord passes from it 

 ventrally on either side of the oesophagus to unite with a 

 ganglion lying below the oesophagus in the second metamere, 

 forming the circumcesophageal commissure. To the subcesopha- 

 geal ganglion of the second metamere there succeeds a 

 pair of ganglia in each metamere, each pair being united 

 with the preceding and succeeding pairs by two longitudinal 

 cords of nerve-fibres, the connectives, the whole constituting 

 the ventral nerve-chain, and furthermore the ganglia of each 

 pair are united by a transverse commissure. The ventral 

 nerve-chain has therefore a distinctly ladder-like arrange- 

 ment, frequently somewhat obscured, however, by the approxi- 

 mation of the ganglia of each pair and a consequent shorten- 

 ing of the transver.se commissures. From the various ganglia 

 nerves arise which pass to the musculature of the metarneres 

 and to the hypodermis and its sense-organs. In the major- 

 ity of forms the nervous system lies freely in the ccelom 

 surrounded by a special sheath, but occasionally in various 

 forms widely separated genetically from one another, such as 

 Polygordius and the Opheliaceee, it presents a primitive char- 

 acter in being completely -imbedded in the hypodermis, 

 recalling the condition in certain Nemerteans and in the 

 Cnidaria. Special nerves arising from the supracesophageal 

 ganglion are supplied to the walls of the digestive tract, form- 

 ing the so-called stomatogastric nerves. 



Sense-organs of various kinds are of frequent occurrence 

 at different portions of the bod}- of the Polychaeta. In addi- 

 tion to the cephalic and caudal cirri which are richly supplied 

 with nerves and are presumably tactile in function, eyes are 

 of very general occurrence. They are usually situated on the 

 head, sometimes in connection with the hypodermis and 



