Page Four 



EVOLUTION 



October, 1928 



A Lesson From Lop-Sided Crabs 



By S. J. Holmes 



T^HERE is a very odd family of crabs known in the jargon 

 of the zoologist as the Lithodidae. All the members of this 

 family are more or less asymmetrical, or lop-sided. The philo- 

 sophical observer is naturally led to ask "How did they get that 

 way? A specialist on crabs can give a very plausible answer. 



The species of this family are 

 very diverse in form, size and 

 habits. Large spine forms inhabit 

 the deep waters of the ocean. 

 Small species, usually with rather 

 soft bodies, are found among the 

 rocks near the shore. One of these, 

 Hapalogaster, lives under rocks 

 and has the body very much flat- 

 tened as if it had been run over 

 by a heavy roller. Another, Cryp- 

 tolithodes, has the shelly covering 

 of the body drawn out into wing- 

 like expansions that completely 

 cover over the legs. Another is a 

 short, pudgy creature with a form 

 more like that of a typical crab. 

 And there are various others. But 

 amid the great diversity of forms characterizing the members 

 of this group, there are certain well marked resemblances which 

 lead zoologists to class them, without hesitation, in one family. 

 They clearly constitute what is called a natural group. 



In all the species of the family the right pincer is larger and 

 stronger than the left one. The last pair of legs is small, weak, 

 and imperfectly developed. And, most striking of all, the abdo- 

 men is more or less one sided and has appendages only on the 

 left side. In ordinary crabs these appendages are symmetrically 

 situated on both sides of the abdomen. They are used for 

 holding the eggs which are carried about by the females until 

 the young are hatched. In the Lithodidae they perform the same 

 function, although the complete absence of appendages on the 

 right side naturally imposes a handicap. It is difficult to imagine 

 any benefit to the species from such a peculiarity. The same 

 condition is found in the large spiny denizens of deep water, 

 the much flattened Hapalogasters, the highly modified, winged 

 Cryptolithodes, and all the other species of this family. Why 

 should all the species have the right pincer larger than t.'ie left, 

 while in other asymmetrical crustaceans the larger pincer is found 

 indifferently on either side of the body? The special cre<;tioni6t 

 must assume that God endowed these poor crabs with the same 

 kind of a handicap, quite regardless of their habits and tlie 

 peculiar features of their environment. The arrangement could 

 hardly be imagined to be of any use to them. 



Hermit crabs. A Pagurus Hirsutlusculus, in a coiled shell. 

 B. Pagurus Samuells, removed from its shell. Note the 

 egg-bearing appendages on the left side of the abdomen, 

 and the larger right pincer. 



Courtesy University of California 



But leaving the creationist to wrestle with the difficulty let us 

 consider what light may be shed by the theory of evolution. 

 According to this theory the resemblances found in the members 

 of a natural group owe their existance to the one cause of 

 likeness among living creatures; descent from a common ancestor. 



In his endeavor to explain how 

 the Lithodidae gained their pecu- 

 liar one-sided condition the evo- 

 lutionist would naturally look 

 to the most closely related group, 

 the hermit crabs. Most species of 

 hermits have the peculiar habit of 

 living in the coiled shell of mol. 

 lusks into which they insert the 

 abdomen. They present some ob- 

 vious adaptations to this mode of 

 life. UsuaUy the pincers are un- 

 equally developed; the abdomen is 

 soft and more or less twisted to 

 fit the coil of the shell, and in the 

 female is commonly furnished with 

 egg-bearing appendages only on 

 the left side. The posterior walk- 

 ing legs are also small and weak, since they are usually partly 

 enclosed in the shell. They show a marked asymmetrical structure. 

 Individuals frequently leave their shells, especially when they 

 outgrow them, but they usually find a new one to inhabit before 

 leaving their old domicile. The soft abdomen would be likely to 

 fare ill without its customary protection. 



The student who attentively compares the Lithodidae with the 

 hermits will be struck with the many close resemblances. He 

 could scarcely fail to come to the conclusion that lop-sidedness 

 of the Litliodidae is a trait inherited from their hermit ancestors 

 which owed it to their habit of living within coiled shells. The 

 Lithodidae are hermit crabs which have left their shells and 

 taken again to a free life. When we consider the close resemb- 

 lances between these two groups, and the fact that the smaller 

 and more primitive of the Lithodidae resemble the hermits most 

 closely, this is the only rational interpretation that can be made. 

 There are few cases in the organic world where the morphological 

 evidence for descent is so peculiarly cogent. 



Only through critically observing and comparing the structural 

 characteristics of related organisms can one gain a really adequate 

 conception of the morphological evidence for descent. Practically 

 all who have qualified themselves to have an opinion by going 

 through this kind of discipline have come to the same conclusion 

 as to the fact of organic evolution. 





Various descendants of Hermit crab. Note larger riglit pincers. C. Hapalogaster cavicauda. Hairs removed from left legs and pincer. 

 Common under rocks at low tide. D. One of larger species of Lithodidae from deep water off California coast. E. Cryptolitbodes 

 sitchensis, from below. Male; abdomen therefore shows little twist. CoiTtesy University of California 



