gradually-degraded forms which eventually 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 altogether. It is curious, however, that the same modified form of radula should appear in species of _Ovula_ (_e.g._ _ovum_) and that the same absence of radula should occur in species of _Eulima_ (_e.g._ _polita_) 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 commensalism, or the habitual association of two organisms, as distinguished from parasitism, where one form preys more or less upon the other. Mr. J. T. Marshall has given[178] an interesting account of the association of _Montacuta ferruginosa_ with _Echinocardium cordatum_. 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 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 Rev. Dr. Norman has noted[179] a somewhat similar habitat for _Lepton squamosum_. This rare little British species was found at Salcombe, living in the burrows of _Gebia stellata_, in all probability feeding upon the secretions from the body of the crustacean. Dr. Norman suggests that the extreme flatness of the shell of the _Lepton_ is of great advantage in enabling it not to get in the way of the _Gebia_ as he scuttles up and down his burrow. Another species of _Lepton_ is found on the coast of Florida in a precisely similar locality,[180] while a third species, occurring on the Oregon and California coasts, actually attaches itself to the inner surface of the abdomen of a _Gebia_.[181] [Illustration: FIG. 32.--_Ephippodonta Macdougalli_ Tate, S. Australia. =A=, Burrow of prawn, the X indicating the position of the mollusc; _sp_, sponge. =B=, Ventral view of _Ephippodonta_; _by_, byssus; _f_, foot; _m_, mantle; _mm_, fused mantle borders. =C=, View of interior of shells; _h_, hinge; m´m´, adductor muscles. (=A= × ½; =B= and =C= × 2.)]