WOE MS TO VERTEBRATES 



63 



tubes. Transverse vessels run from these to all parts 

 of the body. The dorsal tube pulsates and thus acts 

 as a heart. The surface of the body no longer suffices 

 to gather oxygen, hence we find special feathery gills 

 on the parapodia. But these gills are merely expanded 



8. CKOSS-SECTION OP BODY SEGMENT OF ANNELID. LANG. 



dp and vp, dorsal and ventral halves of parapodia ; b and ac, bristles ; k, gill ; dc 

 and vc, feelers ; rm, lateral muscles ; Im, longitudinal muscles ; vd, dorsal 

 blood-vessel ; vo, ventral blood-vessel ; bm, ventral ganglion ; ov, ovary ; tr, 

 opening of nephridium in the perivisceral cavity; np, tubular portion of 

 nephridium. The circles containing dots represent eggs floating in the peri- 

 visceral fluid. 



portions of the body wall, arranged so as to offer the 

 greatest possible amount of surface where the capil- 

 laries of the blood system can be almost immediately 

 in contact with the surrounding water. 



The nervous system consists of a large supra-ceso- 

 phageal ganglion in the first segment ; then of a chain 

 of ganglia, one to each segment, on the ventral side of 

 the body. With one ganglion in each segment there is 



