CRUSTACEANS IN CH^TOPTERUS TUIJES. I I I 



sluggish it showed similar sensitiveness to shadows. Such 

 reactions would of course help to protect these animals from 

 predaceous enemies. 



On July 24 one of the Pinnixa in the "dark box" liberated a 

 number of zoea larvae which swam persistently against the glass 

 toward the light for an entire day; they were then removed. 



PinnixcB underwent ecdysis in the dishes in which they were 

 kept; one shed on July 24 and two on August 5 (Fig. 6). They 

 were apparently much hardier than Polyonyx and did not throw 

 off their legs readily nor die if the water became foul. Polyonyx, 

 like all its near relatives (Caiman, 'n, p. 114), readily practices 

 autotomy, and quickly succumbs to unfavorable conditions. 

 Its extreme cleanliness has already been mentioned. 



GENERAL DISCUSSION. 



Perhaps the chief point of interest in these dissimilar crus- 

 taceans which have come to be associated with Chtztopterus so 

 closely that they are rarely found elsewhere lies in the similarities 

 in physiology and structure which have enabled them to take 

 up such a peculiar mode of life. The similarities ought to point 

 to essential or fundamental characteristics from an ecological 

 point of view, and the unlike features should be of less importance- 



Let us first examine the differences between Polyonyx and 

 Pinnixa. As to relationship, both are decapods, but the former 

 belongs to the family Porcellanida 1 (tribe Galatheidea, section 

 Anomura) and is therefore not a crab despite its general appear- 

 ance; the latter is a true crab of the family Pinnotheridae (tribe 

 Brachygnatha, section Brachyura). The Porcellanidae use only 

 three pairs of legs for walking, the first forming the chelae and 

 the last being very small and carried folded up at the sides of 

 the body or even within the gill chambers. They are mostly 

 found under stones along the sea .shore and among corals. The 

 family Pinnotheridae contains many crabs which live as com- 

 mensals in molluscs, worms, sea-urchins, and other animals. 

 Their shells are often softened or membranaceous, and their eyes 

 are very small; both these characteristics have been supposed 

 to have arisen as a result of commensal life (Stebbing, '93, p. 100) ; 

 the fifth legs are much shorter than the fourth but are used for 



