MOLLUSCS. 



343 



parasite, and it. is a curious fact tliat all the parasitic Mollusca affect only the echino- 

 denns and coslenterates. On our New England coast aS'. stimpsoni occurs on the com- 

 mon sea-urchin, Strongylocentrotus; in England Stylina turtoni also affects sea-urchins ; 

 a Mediterranean species lives firmly attached to the anal tube of the feather star, Com- 

 atula; while, to mention but one more species, S. aster icola, as its name implies, dwells 

 on starfish. Some of these forms are true parasites. With their slender foot they obtain 

 an entrance between the calcareous plates of the starfish or sea-urchin, and there they 

 live on the juices of their host, only the apex of the shell being visible from the 

 exterior. 



In Eulima the parasitic habit is carried still farther. Most of these live on liolo- 

 thurians, .although some attack starfishes. They even enter the alimentary tract of 

 some of the sea-cucumbers, where they feed on the food of tlie host. One species has 

 become so modified by parasitism that, though it still retains a shell, it has lost many 

 of its distinctive molluscan characters. This species, as described by Sempei', lives 

 on the outside of a species of holothurian, and its proboscis is enormously developed 

 so tliat it jsierces the tissues of its host and enters the coelomatic cavity. Feeding 

 thus on the fluids of the sea-cucumber, it has no need for teeth, and hence its lingual 

 ribbon has disappeared, and from the same cause, disuse, the foot and eyes have fol- 

 lowed a like course. Our American Eulima oleacea does not seem to have carried its 

 parasitic habits so far. It occurs on the skin of Thijone briareus, a common holothu- 

 rian south of Cape Cod. 



It is doubtless the fact that it li\-es a ])arasitic life that has led some naturalists to 

 place that most degraded of molluscs, Entoconcha, in this family. Not enough is 

 known of it to warrant such a position on any other ground. It is referred to on a 

 previous page (p. 297). 



The TuRRiTELLiD^E embraces forms with a shell much like that of the Pyramidelli- 

 dffi ; long and turreted, with a round mouth which is not notched or produced into a 

 canal. The operculum is horny and many-whorled. The animal has a moderate, but 

 short foot, a short muzzle, and the 

 eyes are placed at the outer base of 

 the tentacles. The margin of the 

 mantle is fringed. Nearly a lain 

 dred species are known, all from 

 salt water. Most of them are cov 

 ered with a brownish epidermis. 



Closely allied to the last faniii_\ 

 is the Vermetid^s, some of the mem 

 bers of which recall the Serpula 

 among the worms (p. 227) on ac 

 coimt of the irregularity and vei 

 mian shape of the shells. The ani 

 nial has an elongate head bearing 

 two long tentacles. The shell be 

 gins as a regular spiral, but after a 

 few turns the resciilaritv is lost, and -^ , , 



'^ ; . i 1 lil I u nt.tabUm.biu.alu 



the tubular shell grows in any dn-ec- 



tion, the spiral character almost entirely disaiijiearing. Tlie shell grows more rayiidly 



than the animal, and, as a result, the posterior portions of the tube are partitioned 



