86 TEE ZOOLOGIST, 



phinus), was suddenly lifted in order that it might be ascertained 

 what the echinoderm was doing ; and it was then found that the 

 latter was so firmly affixed by spines, tube-feet (and pedi- 

 cellarise?), that the Top, which was dead, was dragged from 

 its shell. The individuals of the Dog-Whelk (Nassa reticulata), 

 some sixty in number, and of the Periwinkle (Littorina littorea), 

 some fifteen in number, which have from time to time been 

 placed in the tank containing the twelve Sea-Urchins, have 

 been much persecuted by the echinoids. The operculum of one 

 such Periwinkle, which was rescued from a Sea-Urchin, had 

 been nibbled around its edge, no doubt during an attempt of the 

 Sea-Urchin to reach the soft body of the mollusc. In the case 

 of a gastropod, which of course is able to withdraw into its 

 shell for some distance, the extensile mouth-membrane and the 

 protrusible teeth and lip of the Sea-Urchin must be of great use ; 

 but it is impossible to observe directly what happens, owing 

 to the close contact between the lower surface of the Sea-Urchin 

 and the mouth of the shell. A Sea-Urchin may be seen from 

 time to time to climb slowly up the glass, dragging with it, and 

 eating, the body (without the shell) of a Periwinkle or a Dog- 

 Whelk. 



The Sea-Urchins have also eaten the eggs laid by Dog- 

 Whelks. On two occasions they have been seen to devour 

 the byssal threads abandoned by an Edible Mussel (Mytilus 

 edulis) . 



A Sea-Urchin will sometimes place itself in such a position 

 that it can firmly hold a living mollusc, and will then repeatedly 

 scrape the surface of the shell with its teeth, in some cases 

 apparently in order to eat the hydroids, polyzoans, or seaweeds 

 which grow upon the shell, and in other cases to eat the peri- 

 ostracum. This has been observed in the case of the Edible 

 Mussel, Slipper Limpet (Crepidula fornicata) , Dog-Whelk, Sting- 

 Winkle (Murex erinaceus), and Dog- Winkle ; and the rugosities 

 of the shells of the Sting-Winkle and Dog- Whelk may be 

 scraped in this way until they become worn down to the level of 

 the rest of the shell (fig. 1, a). In doing this the Sea-Urchin 

 may desire to reach and eat the shell, as well as to obtain the 

 materials which invest it. Indeed, the Purple-tipped Sea- 

 Urchin will eat comparatively large quantities of shell. The 



