282 lyitrodudion to Animal Morphology. 



the operculum.* The front edge has, in some, two 

 lip-like, often glandular, processes. The foot is rudi- 

 mental in Glaucus, or absent (Phyllirhoe, Vermetus, 

 &c.), or its sole may be reduced to a sucker, as in the 

 males of some Heteropoda. 



The dorsal (haemal) area of the skin behind the 

 head usually forms a mantle with lateral folds, secret- 

 ing from its surface, but not by special glands, a 

 shell, but both disappear ; shell and mantle may 

 vanish early, and the dorsal integument may show 

 instead, papillae (^olisf), or branched processes (Den- 

 dronotus), or cirri (Tethys). The mantle may be 

 margined by a series of tentacles (Haliotis, Parmo- 

 phorus) ; its edge is thick and muscular, frequently 

 fringed with pigment-glands secreting the colours of 

 the shell. The edge alone secretes the prismatic ele- 

 ment of the shell. The surface secretes the nacreous 

 lining. The shell covers the dorsal side of the 

 animals, and is homologous with the pair of valves 

 and ligament of a bivalve, or with the anterior valve 

 of a Brachiopod ; it may be of two kinds, either 

 symmetrical, limpet-like (and then may have a slightly 

 coiled apex, as in Calyptr£ea\ or an inner shelf as in 

 Calyptraea, Crepidula, &c., or a perforate apex as in 

 Fissurella, or a hinder notch as in Emarginula. The 

 aberrant Chiton shell is also symmetrical. In others the 

 shell is coiled, and then may be globose (Natica), sub- 

 globose, oval, conical, subulate, fusiform, pyriform, 



* In Harpa the metapodial lobes are large, and overlap the body shell. 

 The dermal area referred to here is called the opercular mantle ; it may 

 exist without an operculum, as in Marginella ; it may have a coUar of ten- 

 tacular processes round it (Turbo, Ampullaria). 



t These bear cnidse. No such weapons exist in the Proctonotidae : but 

 these have terminal openings to their papillae. 



