86 LIFE BY THE SEASHORE. 



ing the parapodia, or "feet." Of these each segment bears 

 a pair, and their structure is somewhat complicated (see Fig. 



29). Each consists of a dorsal 

 and ventral process, both bear- 

 ing tufts of stiff bristles. More 

 careful examination by means 

 of a lens will show in addition 

 the following points. Both 

 dorsal and ventral processes 

 t, or parapodium, of are Globed, and it is the lower 



Nereis pelagica. d, dorsal cirrus ; lobe of the dorsal and the Upper 



WiS\WS*S: ^be of the ventral only which 

 relative lengths of the d.fferent bear bristles, the other two 



parts, and especially the long , , IT, 



dorsal cirrus, are distinguishing lobes are mere vascular plates. 



Ehters. f the Spedes ' After Further, both processes give off 

 slender sensitive outgrowths, 



the feelers, or cirri, of which one is dorsal and the other 

 ventral. The bristles have usually a peculiar golden sheen, 

 and in each tuft there is one of needle-like shape which 

 only projects very slightly, but which is easily found on 

 dissection. It is to these needles that the muscles are 

 attached, and they form, as it were, the skeleton of the foot. 

 To recapitulate, the parapodia are hollow, muscular out- 

 growths of the lateral body- wall; they are divided into 

 bilobed dorsal and ventral processes, each bearing bristles, 

 each giving off a delicate sensitive cirrus. By virtue of 

 their muscles and bristles the parapodia are locomotor 

 organs ; by virtue of their contained blood-vessels they are 

 respiratory organs; by virtue of their sensitive cirri they 

 are sense-organs. As one or other of these three functions 

 predominates in the bristle-worms, we have a corresponding 

 variation in the structure of the foot. 



If we look now at the anterior region or head, we find 

 that it differs considerably from the other parts of the body. 

 Overhanging the mouth is a dorsal lobe which bears eyes 

 and tactile processes, and is the head proper. The lobe is 

 called the prostomium, and is probably not equivalent to 

 a segment. Surrounding the mouth is the peristomium, or 

 first true segment, which also bears tactile processes, but 

 has no parapodia nor bristles. In most bristle-worms the 

 head region consists of these two parts, but in a few, other 



