216 TRANSACTIONS OF THE [mAR. 8, 



" When the hermit and crab and the annelid were taken out 

 of the shell and both left in an aquarium in which there were 

 empty snail shells, the worm did not deign to enter any until the 

 crab had made its choice and taken possession of one. Then it 

 immediately crawled up to and took its place in the same, with- 

 out any hindrance on the part of the crab. Yet, without doubt 

 the crab could have seized and devoured this worm as well as it 

 had a Nereis diversicolor on a previous occasion." The writer 

 suggests that beside stinging and killing prey, the worm may 

 also free the crab from larvae of parasitic isopods or Rhizoceph- 

 ala. That this last mission is not always accomplished is in- 

 dicated by my having found on the companion of Nereis cyclurus 

 a large and flourishing Argeia. The fact that the hermit-crab 

 could distinguish between the two species N. fucata and N. 

 diversicolor might be explained by the distastefulness of the 

 former. From the above evidence, however, it seems safe to 

 assume that tUe oommensalism is mutualistic. In confinement, 

 the Nereid outlives the crab, by many days. 



III. Structural Modifications arising from Commensal Life. 



1. The posterior two-thirds of the worm being no longer needed 

 for locomotion and protection, degeneration has taken place in 

 the cuticular and muscular layers of this region as in N. fucata 

 /5 inquilina. There is further modification in the size and 

 strength of the bristles. 



2. The body is pigmented only as far as the twenty-seventh 

 somite and on the dorsal side alone. Wiren observes that this 

 surface is all that is ever exposed to light. 



3. In spite of the degeneration of the posterior part of tlie 

 body, the respiratory lobes on the parapodia of this region are 

 fully developed. In the normal extended position assumed by 

 worm and crab there is a constant current of water passing from 

 the gills of the latter along these posterior appendages of the 

 Nereid. 



4. The fact that of the adults, females alone are found, may be 

 due to the eflfect of a highly nutritive and well-protected mode 

 of life. That phj'siological factors have not a little to do with 

 the determination of sex in annelids has been suggested by 

 Wheeler in the account of the sexual phases of Myzostoma* 

 N. dumerillii possesses at least five different mature forms, 

 some Heteronereis, some not, some living on the bottom, some 

 at the surface. The relations of these forms to each other and 

 the causes of sexual modifications, might by experiment be 

 shown to depend upon different physiological conditions. 



*Mittheil. aus ZoOl. Stat. Napl., Vol. 15, 1896. 



