ON AN INSTANCE OF COMMENSALISM. 983 



46. On an Instance of Commensalism between a Hermit 

 Crab and a Folyzoon. By R. Kiekpateick, F.Z.S., 

 and Dr. J. Mbtzelaae. 



[Received August 4, 1922 : Read October 24, 1922.] 

 (Plates I., II.*) 



General. — In the year 1906, Mr. F. P. Vermeulen, Ymuiden, 

 Holland, started with a fishery expedition to the West African 

 shores with the object of seeing if the "langusts" of Cape Blanco 

 and its neighbourhood would be worth exploitation. The rich 

 material collected by him was given to the Zoological Society, 

 " ISTatura Artis Magistra," at Amsterdam. It contains many 

 Hermit Crabs, the commonest of which is Petrochirxis granuli- 

 manus Miers. 



The younger specimens of this Crustacean live in small shells 

 of Turritella hrevialis, Natica fuhninea, Nassa miga, Terehra 

 senegalensis, Aporrhais pes pelecani^ and Dorsanum politum. These 

 samples come from Cape Blanco and the large Greyhound Bay 

 (" Bale du Levrier ") close by, from depths not exceeding 

 •iO metres. A few specimens were also met with on the Senegal 

 coast. 



Kow, some of these "youngster" home-shells show a thin 

 incrustation of a Polyzoon, a few layers thick near the orifice 

 and only one or two layers over the rest of the surface (PI, I. fig. 3), 



If we compare the older young and adult pagurids with the 

 former, we notice the said colony has inci-eased very much, 

 resulting in a heavy turnip- or potato-like spheroidal mass, 

 about 6 cm. in diameter (fig. 1), completely involving the original 

 contour of the shell. At one side there is a funnel, about 2 cm. 

 wide externally and filled in by the heavy claw of the Petro- 

 chirus. A section (fig. 2) shows the gasteropod shell covered 

 entirely by a dense, stony crust of the Polyzoon in. numerous 

 layers, amounting to 56. The shell-substance does not appear 

 to have been eaten away as in shells encrusted by Lepralia edax. 

 The free margin of the orifice of the larger specimens is made 

 up wholly of Polyzoan layers folded on themselves (fig. 2). On 

 dissection, the calcareous Polyzoan mass appears to be homo- 

 geneous, excepting that a few barnacles have been overwhelmed 

 and sufi'ocated. The most sound colonies have a perfectly smooth 

 surface of a deep violet-brown colour in spirit, and they form a 

 typical feature of the fauna of the Cape Blanco coast. But for 

 reasons not yet explained the colony may lose its power of 

 resistance against destructive forces of the environment. Every- 

 one knows the difi"erence between "living" mollusc-shells so 

 eagerlj^ wished for by conchologists, and the porous, brittle shells 

 the legal inhabitants of which have been replaced by boring 

 * For explanation of tlie Plates, see p. 990. 



66* 



