MONTACUTA. 207 



favoured with its company. An ally, if not a variety, 

 of the present species of Montacuta was described by 

 Recluz in the ' Revue de Zoologie' for 1844 under the 

 name of Erycina seminulum, and stated to have been 

 discovered by M. Caron in the Mediterranean on the 

 spines of different kinds of sea-urchins, principally 

 Echinus esculentus. I mention these cases to show that 

 it is not confined to S. purpureas, or the " purple-heart 

 urchin," as is commonly supposed. 



The byssal threads by which this curious mollusk 

 attaches itself are exceedingly strong and coarse. Mr. 

 Clark observed it in active motion after he had sepa- 

 rated it, still adhering to the spines, from a Spatangus. 

 He says, "When the animal marches, its foot is ex- 

 tended, and its rounded termination is instantly fixed 

 to the vase in which it is deposited ; then by the re- 

 tractor muscle it is drawn forward, making such rapid 

 progression as to cross a watch-glass in a minute, and on 

 the passage turns itself several times by a twist of the 

 foot from side to side." In another description which 

 he has given of this operation, he uses the appropriate 

 expression " iterumque, iterumque," to show the toil- 

 some march of the little creature, which evidently would 

 have preferred remaining on the sea-egg from which it 

 was so unceremoniously dislodged to taking a long 

 walk across and round Mr. Clark's watch-glass, in a 

 vain search for fresh quarters of the accustomed sort. 

 The gills and green liver are visible through the shell 

 in some specimens which are more transparent than 

 others, the former crossing it diagonally. The shape 

 and position of the cartilage is very remarkable. Some- 

 times the shell is partly incrusted with a ferruginous 

 deposit. The number of fry, with their shells com- 

 pletely formed, which are found in some individuals, is 



