On _Tunicata_.--_Lamellaria_ deposits its eggs and lives on an Ascidian (_Leptoclinum_), and the common _Modiolaria marmorata_ lives in colonies imbedded in the test of _Ascidia mentula_ and other simple Ascidians. [Illustration: FIG. 31.--Two species of _Eulima_: =A= is sessile on the skin of a Holothurian, through which it plunges its sucking proboscis (_Pr_); =B= creeps freely in the stomach of a Holothurian. (After K. Semper.)] Special points of interest with regard to parasitic Mollusca relate to (1) _Colour_. This is in most cases absent, the shell being of a uniform hyaline or milky white. This may be due, in the case of the endo-parasitic forms, to absence of light, and possibly, in those living outside their host, to some deficiency in the nutritive material. A colourless shell is not necessarily protective, for though a transparent shell might evade detection, a milk-white hue would probably be conspicuous. (2) _Modifications of structure._ These are in many cases considerable. _Entoconcha_ and _Entocolax_ have no respiratory or circulatory organs, and no known nervous system; _Thyca_ and certain _Stilifer_ possess a curious suctorial apparatus; the foot in many cases has aborted, since the necessity for locomotion is reduced to a minimum, and its place is supplied by an enormous development of the proboscis, which enables the creature to provide itself with nutriment without shifting its position. K. Semper notices a case where a _Eulima_, whose habitat is the stomach of a Holothurian, retains the foot unmodified, while a species occurring on the outer skin, but provided with a long proboscis, has lost its foot altogether.[177] Special provision for holding on is noticed in certain cases, reminding us of similar provision in human parasites. Eyes are frequently, but not always wanting, even in endo-parasitic forms. A specially interesting modification of structure occurs in (3) the _Radula_ or ribbon-shaped arrangement of the teeth. In most cases of parasitism (_Eulima_, _Stilifer_, _Odostomia_, _Entoconcha_, _Entocolax_, _Magilus_, _Coralliophila_, _Leptoconcha_) it is absent altogether. In _Ovula_ and _Pedicularia_, genera which are in all other respects closely allied to _Cypraea_, the radula exhibits marked differences from the typical radula of the Cypraeidae. The formula (3·1·3) remains the same, but the laterals are greatly produced and become fimbriated, sometimes at the extremity only, sometimes along the whole length. A very similar modification occurs in the radula of _Sistrum spectrum_ Reeve, a species which is known to live parasitically on one of the branching corals. Here the laterals differ from those of the typical _Purpuridae_ in being very long and curved at the extremity. The general effect of these modifications appears to be the production of a radula rather of the type of the vegetable-feeding _Trochidae_, which may perhaps be regarded as a link in the chain of