OTHER CRUSTACEA 335 



anterior part of the head region and the telson. A carapace, 

 resembling that of the crayfish, is variously developed in different 

 species. The internal organization is Hkewise modified in accord- 

 ance with the habits of particular species and the external features 

 are highly speciahzed in many instances, yet the fundamental 

 plan remains. There is, in fact, no better place in the Animal 

 Kingdom for illustrating how an underljdng unity of organization 

 may pervade a seeming diversity of structure in adaptation to a 

 wide range of functions. Just as the crayfish or any other mala- 



FiG. 163. — A hermit crab encaced in a sponge. 



The crustacean, a species of Pagurus from Puget Sound, like other so-called "hermit 

 crabs" appropriates the empty shell of a gastropod mollusc. The sponge becomes attached 

 to this shell and grows completely around it, eventually destroying the shell itself and leaving 

 the crab in a spiral cavity that was originally the cavity of the shell. The association 

 seems to be a normal one and, therefore, a case of symbiosis or living together with mutual 

 advantage, since the crab is protected while the sponge, which is a sessile animal, is dragged 

 from place to place. A, the crab exposed as it appears during locomotion, and the encasing 

 sponge in which there are two oscula or exhalent openings. B, the sponge cut open showing 

 part of the spiral cavity occupied by the crab, the many small internal cavities of the sponge, 

 and a single osculum. (Drawn by George T. Kline.) 



costracan offers an unusual opportunity for study of the serial 

 homology that finds expression in the body of a single animal, 

 so the various types of Malacostraca offer remarkable examples 

 of the homologies between corresponding parts in different 



animals. 



The Entomostraca. — The Entomostraca include the simplest 

 forms among the Crustacea, although many entomostracans are 



