COLOUR OF PARASITIC MOLLUSCA 



79 



in the nutritive material. A colourless shell is not necessarily 

 protective, for though a transparent sh(41 niinht evade detection, 

 a milk-white hue would prohably Ijo conspicuous. (2) ModiJic<(- 

 tions of structure. These are in many cases consideral»le. Ento- 

 conclia and Untocolax have no respiratory or circulatory organs, 

 and no known nervous system ; Tliyca 

 and certain Stilifcr 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 l)y an enormous 

 development of the proboscis, which en- 

 al)les the creature to provide itself with 

 nutriment without sliiftiug its position. 

 K. Semper notices a case where a Uulima, 

 whose habitat is the stomach of a Holo- 

 thurian, retains the foot unmodified, while 

 a species occurring on the outer skin, 

 but provided with a long proboscis, has 

 lost its foot altogether.^ Special pro- 

 vision for holding on is noticed in cer- 

 tain cases, reminding us of similar provision in human para- 

 sites. Eyes are frecpiently, l)ut not always wanting, even in 

 endo- parasitic forms. A specially interesting modification of 

 structure occurs in (3) the Badida or ribl)on-shaped arrangement 

 of tlie teeth. In most cases of parasitism {Etdlma, JSfiNfcr, 

 Odostomia, Untoconcha, Entocolax, Magilus, Corcdlio].)]iila, Lepto- 

 conclui) it is al)sent altogether. In Ovula and Fedicidaria, genera 

 which are in all other respects closely allied to Cy2yi'ara, tlie 

 radida exhil)its marked difl'erences from the typical radula of 

 the Cypraeidae. The formula (^M"']) remains the same, but the 

 laterals are greatly produced and l)ecome find»riated, sometimes 

 at the extremity only, sometimes along the wdiole length. A 

 very similar modification occurs in the radula oi. Sisti'iim spectrum 

 lieeve, a species wliich is known to live parasitically on one of 

 the branching corals. Here the laterals differ from those of 

 the typical Furpuridac in being very long and curved at the 

 extremity. Tlie general (dfect of these modifications appears to 

 be the prijduction of a radula rather of the type of tlie vegetable- 

 ^ Animal Life, p. 351. 



Fig. 31. — Two species of Eu- 

 lima : A is sessile on the 

 slvin of a Holotliurian, 

 tlirough which it i^lunges 

 its sucking proboscis {Fr) ; 

 B creeps freely in the 

 stomach of a Holotliurian. 

 (After K. Semper. ) 



