1918] Esscnberg: Distvihution of the Folynoidac 221 



Mcintosh (1900) has observed a remarkable variation in Fohjnoe 

 scolopendnna. This worm lives as a commensal and as a free-living 

 form around the shores of England and off the Hebrides. Besides 

 the remarkable differences in size between the free-living and com- 

 mensal forms from the shores of Great Britain and the individuals 

 off the Hebrides there is a difference in coloration particular to each 

 group. The coloration is especially conspicuous in the commensals 

 occupying the burrows of Lysidice. The commensal, according to 

 Mcintosh's observation, is very narrowly compressed. The pigmenta- 

 tion of the anterior ventral portion of the body of the worm and 

 around the mouth has a coloration mimicking the Lysidice. The tube- 

 dwelling Polynoc scolopendnna have the setae greatly modified; espe- 

 cially is this true of the dorsal setae, the tips of which are curiously 

 wrinkled. The specimens from the Hebrides, however, prove that 

 in the normal conditions the setae are finely tapered. The wrinkled 

 condition has evidently been brought about through the commensal 

 life. 



E. GENERAL DISCUSSION 



The fact that Polynoidae fall into different groups according to 

 their geographical and bathymetrical distribution suggests that the 

 phenomenon of distribution is governed by certain physical and 

 chemical factors and that the Polynoidae, reacting in response to 

 the external stimuli, are limited to their particular distributional 

 areas. 



One of the most important factors controlling distribution is the 

 temperature. This is proved by the fact that certain species of Poly- 

 noidae are limited to definite temperature zones, and that boreal 

 species, occupying the littoral zone, occur as deep water species in 

 temperate and in subtropical zones. Since the most apparent simi- 

 larity between the boreal shallow waters and the subtropical deep 

 waters consists in the temperature, this latter may be considered as 

 the chief factor in distribution. Other factors, as currents and winds, 

 may act in transportation of the larval and adult forms or by influenc- 

 ing the temperature and the food conditions. 



The food conditions may be of importance in distribution and 

 polynoids may be naturally expected to be more abundant where food 

 conditions are maximum. This condition is found in the littoral zones. 



