COLOUR OF PARASITIC MOLLUSC A 



79 



tective, for though ;i transparent shell might evade detection, a 

 milk-white hue would probably be conspicuous. (2) Modifica- 

 tions of structure. These are in many cases considerable. JEnto- 

 concha 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 en- 

 ables the creature to provide itself with 

 nutriment without shifting its position. 

 K. Semper notices a case where a Eu- 

 lima, whose habitat is the stomach of a Fm.31.-Two species of ^w- 



. . IP T r> 1 lima: A is sessile on the skin 



Holothurian, retains the foot unmodified, 



of a Holothurian, through 

 which it plunges its sucking 

 proboscis (Pr); B creeps 

 freely in the stomach of 

 a Holothurian. (After K. 

 Semper.) 



while a species occurring on the outer 

 skin, but provided with a long proboscis, 

 has lost its foot altogether.^ Special 

 provision for holding on is noticed in 

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

 sites. Eyes are frequently, but not always wanting, even in 

 en do-parasitic forms. A specially interesting modification of 

 structure occurs in (3) the Radula or ribbon-shaped arrange- 

 ment of the teeth. In most cases of parasitism QEidima^ Stilifer^ 

 Odostomia^ EntoconcJia^ Entocolax^ Magilus, Coralliophila^ Lepto- 

 concha) 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, some- 

 times 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 ap- 

 pears to be the production of a radula rather of the type of the 

 vegetable-feeding Trocliidae^ which may perhaps be regarded as 

 1 Animal Life^ p. 351. 



