CLARK: THE ECHINODERMS OF PERU. 347 



more or less distorted by the presence of a parasitic crab (Fabia chilensis Dana). 

 In specimens of gibbosus, less than ten millimeters in diameter, there is no evi- 

 dence of the presence of the crab ; the test is symmetrical and all of the ocular 

 plates are excluded (by the large genitals) from the periproct. In larger speci- 

 mens, however, the abactinal system shows the effect of the parasite, and in adults 

 the whole periproct and the ring of genital and ocular plates are more or less 

 profoundly modified. The number of ocular plates in contact with the periproct 

 ranges, in adults, from one to five, but as a rule the anterior ocular and the two 

 of the left side are in contact, while the two on the right side are exsert. Very 

 rarely one sees an adult gibbosus which appears to be free from the parasite, but 

 even in such specimens it is the left side which has the insert oculars. Among 

 the many questions which arise in connection with this interesting case of sym- 

 biosis, are these : Is this symbiosis, with some mutual advantage, or is it pure 

 parasitism ? Is it only the urchins in shallow water, near shore, or also individ- 

 uals in deep water, which are attacked by the crab? At what stage of its own 

 development does the crab enter the urchin ? How does it enter and how does it 

 avoid being cast off ? Does the presence of the crab cause the ultimate death of 

 its host ? Does the crab leave a dead host or does it die too ? How does the 

 crab distinguish gibbosus from S. albus and other Echini ? 



The adult gibbosus may be 60 mm. in diameter, but the great majority of spec- 

 imens seen are under 50. The green coloration is often modified by red (or red- 

 dish brown) tips on the spines. This species seems to be most common in the 

 region between Payta and Callao, but there is a small specimen in the M. C. Z. 

 collection labeled " Valparaiso," and it is probable that the range extends from 

 the Gulf of Guayaquil southwards at least along the shores of northern Chile. 

 Dr. Coker took specimens " with short spines of olive-green color, off northeast 

 side of San Lorenzo Island, in about 2 fathoms " ; others " from the rocks be- 

 tween tide-lines, northeast end of Ferrol Bay, Chimbote, March 1," 1907, where 

 they were " abundant " and had the " spines olive-green, reddish at tips " ; 

 others, " olive-green," were " collected from the rocks in two feet of water (at half 

 tide), at Lobos de Afuera, ... on the bay called ' Independencia.' " 



Meissner (1896) makes the rather surprising error of recording this species from 

 Iquique as Strongylocentrotus albus ; he says all the specimens were more or less 

 deformed by the presence of the crab ; the field note quoted calls them " rother 

 seeigel." If it were not for the very full synonymy given, one might regard the 

 error as a slip of the pen ; as it is, it is difficult to explain. 



Strongylocentrotus albus. 



Echinus albus G. I. Molina, 1782. Saggio St. Nat. Chili, p. 175. 

 Strongylocentrotus albus A. Agassiz, 1872. Rev. Ech., pt. 1, p. 162. 



, Plate 12, figure 1. 



This is the largest of the sea-urchins of the west coast of South America, and 

 of special interest because it is the only echinoderm of that region which serves 



