8o CASES OF COMMENSALISM chap. 



a link in the chain of gradually-degraded forms which event- 

 ually terminate in the absence of the organ altogether. The 

 softer the food, the less necessity there is for strong teeth to 

 tear it ; the teeth either become smaller and more numerous, or 

 else longer and more slender, and eventually pass away alto- 

 gether. It is curious, however, that the same modified form of 

 radula should appear in species of Ovula (e.g. ovuni) and that 

 the same absence of radula should occur in species of Eidima 

 (e.g. loolita) known to be not parasitic. This fact perhaps 

 points back to a time when the ancestral forms of each group 

 are parasitic and whose radulae were modified or wanting, the 

 modification or absence of that organ being continued in some 

 of their non-parasitical descendants. 



Commensalism 



Mollusca are concerned in several interesting cases of com- 

 mensalism, or the habitual association of two organisms, as dis- 

 tinguished from parasitism, where one form preys more or less 

 upon the other. 



Mr. J. T. Marshall has given ^ an interesting account of the 

 association of Montacuta ferruginosa with Echinocardium eor- 

 datum. The Echinoderm lives in muddy sand in Torbay, at a 

 depth of about 6 inches, and the Montacuta lives in a burrow 

 leading from its ventral end and running irregularly in a sloping 

 direction for 3 or 4 inches, the burrow, which is made by a 

 current from the Echinoderm, being almost exactly the width 

 of the Montacuta. The Montacuta were alwaj^s arranged in the 

 burrows in order of size, the largest being close to the Echino- 

 derm, and the smallest of a string of about six at the other end 

 of the burrow. In another part of S. Devon, where the sand 

 was soft and sloppy, the Echinocardia rise to the surface and 

 travel alono- the sand ; in this case the Montacuta were attached 

 to their host by means of a byssus, and were dragged along as 

 it travelled. 



The Rev. Dr. Norman has noted ^ a somewhat similar habitat 

 for LejJton squamosum. This rare little British species was 

 found at Salcombe, living in the burrows of G-ehia stellata, in 

 all probability feeding upon the secretions from the body of the 



1 Journ. of Conch, vi. 1891, p. 399. 2 j^^Ji. Mag. N. H. (6) vii. p. 276. 



