112 A. S. PEARSE. 



walking. Pinnixa never walks backwards, while Polyonyx 

 never walks forwards ; but both move sideways. 



Pinnixa is very hardy and can stand foul water, as well as the 

 indiscriminate growth of organisms on its carapace, and does not 

 have a strong respiratory current. Polyonyx is not hardy, but 

 takes every precaution to protect itself. It has a special cleaning 

 appendage; when at rest it stands high on its setal fringe above 

 the dirt that may collect under it; it never burrows; it has a very 

 strong respiratory current which is deflected laterally so as to 

 clear its abode. Apparently Pinnixa can endure great hardship 

 through its great resistance, and Polyonyx has a number of adap- 

 tations to protect itself from contamination. The former could 

 live almost anywhere, the latter is adapted to life in Chcetopterus 

 tubes or other protected situations. 



The similarities between these commensals are as follows: 

 Both are. like most crustaceans, strongly thigmotropic, and creep 

 into crevices or tubes; they become quiet when a shadow passes 

 over them; they feed by "net casting" after the manner of 

 barnacles; both have a very long breeding season, producing one 

 brood after another; and both have the last leg shortened. 

 Their thigmotropism would easily account for their entering 

 Chcetopterus tubes and their feeding habits are admirably suited 

 for the capture of food in such a situation. The quick cessation 

 of motion when stimulated by a decrease in light might protect 

 them from enemies when out of their tube. Enders ( '05) believes 

 that the long breeding period is an adaptation that has arisen as 

 a result of the protected life in the worm tubes. 



Shelford ('n, p. 603) says: "An animal should be associated: 

 first, with breeding conditions; second, with the feeding con- 

 ditions; third, with the conditions affecting shelter." In the 

 present case, in fact in the case of most crustaceans, external 

 conditions are of consequence in breeding only in limiting parents 

 to a general region where the larva may carry on its later develop- 

 ment, for the eggs are carried by the parent for a longer or shorter 

 time. Some crustaceans (Birgus] have probably adapted their 

 reproduction to suit a particular environment. The feeding 

 habits of Polyonyces are such that they might exist anywhere in 

 shallow water. It is apparently for protection that they have 





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