1918] Essenherg: Distribution of the Polynoidae 219 



reached its large size because of its commensalistic habits, but it is 

 very likely that the commensalistic habit aids it in maintaining its 

 large size. The commensal habits evidently are of some advantage 

 to the poljTioids favoring their growth and development. One advan- 

 tage is that the commensals do not have to exert as much energy to 

 obtain their food as do the free-living worms. In thickly populated 

 places the competition among animals must be considerable, hence an 

 animal must exert a great amount of energy in changing location and 

 in pursuit of prey. At the same time it has to be vigilant in guarding 

 its own safety lest it fall a victim to other animals. The free-living 

 Polynoidae usually are found beneath rocks or weeds or in crevices. 

 Occasionally they venture out of their hiding place, but the least dis- 

 turbance causes them to disappear again. Their great activity wall 

 naturally reduce the volume of the body or check its development. 

 On the other hand, the commensal does not have to exert any energy. 

 It is well protected and leads a passive life. Secondly, it obtains an 

 abundant food supply. The writer has watched some of the tube- 

 dwelling annelids many times. They reach out their numerous ten- 

 tacles in great distance forming a circle and capturing any object 

 within their reach conveying it then to the mouth. It is surprising 

 to see the amount of food and material they may convey to the tube. 

 Once the writer destroyed the tube of some tube-dwelling annelids, 

 crumbling it carefully to pieces without injuring the worms which 

 were then put into a small glass dish where the material of their 

 tubes had been crumbled up in fine granules. The worms immediately 

 set to work with their tentacles rolling the small pebbles and the 

 grains of sand toward their bodies and cementing them with some 

 substance which was formed around the body. Within less than two 

 hours the tubes, about two inches in length, were completed. If a 

 terebellid worm is placed in an aquarium where the food supply is 

 insufficient it stretches out its long tentacles covering considerable 

 distance. The tentacles when stretched to the limit are about equal 

 in length to the body of the worm. As soon as one of the tentacles 

 comes in contact with some substance or food particles it immediately 

 contracts and bends, conveying that substance to the mouth. If the 

 poljTioid is in the tube with its head at the entrance it may capture 

 every food particle that is conveyed toward the tube, robbing thus 

 its messmate of its food and receiving an abundance of food for itself, 

 at the same time remaining perfectly quiet. This passive condition 

 and the abundant food supply will naturally result in an increase in 



