8o CASES OF COMMENSALISM chap. 



feeding Trochidac, which may perhaps be regarded as a link in 

 the chain of gradually degraded forms which eventually termi- 

 nate 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 altogether. It is 

 curious, however, that the same modified form of radula should 

 appear in species of Ovuhi (e.g. ovum) and that the same absence 

 of radula should occur in species of Eulimct (e.g. politci) 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-parafeitical 

 descendants. 



Commensalism 



MoUusca 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 MontaciUa fcrruginosa with Echinocardium cor- 

 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 runuing 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 always arranged in the 

 burrows in order of size, the largest being close to the Echinoderm, 

 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 

 along 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 Eev. Dr. Norman has noted ^ a somewhat similar habitat 

 for Lepton squamosum. This rare little British species was found 

 at Salcombe, living in the biurrows of Gehia stcllata, in all prob- 

 ability feeding upon the secretions from the body of the crustacean. 



1 Jourii. of Conch, vi. 1891, p. 399. ^ ^^^jj, j^f^ig. K E. (6) vii. p. 276. 



