GASTEROPODA. 285 



they have no foot ; but that part of their bodies, 

 which, in ordinary Gasteropods, constitutes the tail, 



FlG. 224. VERMETUS. 



is bent forwards until it reaches beyond the head, 

 where it swells out into a protuberance furnished 

 with a thin operculurn ; this serves as a door where- 

 with to close the entrance of the tube when the 

 animal retreats into its shelter. The head of these 

 Mollusks is provided with two tentacula of moderate 

 size, at the bases of which the eyes are situated : the 

 mouth is a simple vertical slit (Fig. 224). 



ORDER SCUTIBRANCHIATA.* 



The animals belonging to this order have their 

 shells very widely open, and frequently not at all 

 spiriform, so that they cover the back, as it were, 

 with a broad shield. 



The Sea-ears (Haliotis) are the most beautiful and 

 richly ornamented of the group ; their shell is slightly 

 spiral at its commencement, but rapidly expands, so 

 * Scutum, a shield ; branchiae, gills shielded-gitts. 



