Miscellaneous, 313 



t. iii. p. 63, 1860) to the genera Heterocyathus, Heteropsammia, and 

 Stephanoseris. This phenomenon of commensalism likewise attracted 

 the attention of Deshayes (' Catalogue des Mollusqnes de Pile de la 

 Reunion,' p. 65, 1863), who considered the commensals of the polyps 

 to be Gastropods with disunited whorls, and formed for them the 

 genus Cryptobia in the family Vermetidae. Deshayes gives a precise 

 description of the respiratory perforations which traverse the skele- 

 ton of the polyps, to terminate at the body of their host ; he observes, 

 moreover, that the shell of the mollusk persists as far as the external 

 orifice in Heteropsammia, but not in Heterocyathus, that its disunited 

 whorls have not the smooth and glossy surface possessed by those 

 which remain in contiguity, and, finally, that there is reason to 

 attribute this difference to a " progressive dissolution of the skeletal 

 matter of the coral, the pores of which would have been filled up by 

 the mollusk." 



Thanks to the rich material which Dr. Jousseaume collected at 

 Aden, and very kindly handed over to me, I have been able to renew 

 the study of this question, and have arrived at the following curious 

 results : — 



The polyps belonging to the genera Heterocyathus and Hetero- 

 psammia attach themselves, probably on emerging from the embry- 

 onic stage, to the empty but always very small shells of various 

 Gastropods ; as soon as they have become fixed they receive as 

 commensals young Gephyreans of the genus Aspidosiphon, which 

 take up their abode in the cavity of the shell and coil themselves 

 into a corresponding spiral. The two commensals then develop 

 simultaneously — the polyp spreading more and more over the shell, 

 which it completely covers, and finally extends beyond ; the worm 

 growing on its part in the shape of a spiral with disunited coils, 

 and producing in the calcareous tissue of the polyp a cavity of the 

 same shape, which prolongs that of the shell and opens to the exte- 

 rior by a rounded orifice. 



Simultaneously with the growth of the polyp and its host the 

 latter secretes a tube in prolongation of that of the shell, but differing 

 from it by being less thick, by its intimate union with the skeleton 

 of the coral, as also by the aspect of its internal surface, which is 

 neither smooth nor lustrous liko that of the shell : in Heteropsammia 

 the tube thus formed extends in the majority of cases to the external 

 orifice; in Heterocyathus, on the other hand, it develops more 

 slowly, and does not reach this aperture. In order to keep itself in 

 direct communication with the respirable medium, the worm 

 dissolves the surrounding calcareous elements, following certain lines 

 normal to its surface, and thus gives rise to linear perforations 

 which serve for the entry and exit of the ambient water. The 

 formation of air-holes of this description is doubtless due to the 

 solvent action exerted upon the calcareous matter by the secretion 

 of certain cutaneous glands. 



The worms commensal with the polyps are provided with a long 

 extensile proboscis and two solid shields, formed by the juxtaposition 

 of corneous pieces. The proboscis terminates in a peribuccal wreath 



